diff --git a/src/abstractdataframe/abstractdataframe.jl b/src/abstractdataframe/abstractdataframe.jl
index 68ad55a54..d7c3365f4 100644
--- a/src/abstractdataframe/abstractdataframe.jl
+++ b/src/abstractdataframe/abstractdataframe.jl
@@ -735,34 +735,36 @@ function _describe(df::AbstractDataFrame, stats::AbstractVector)
     data = DataFrame()
     data.variable = propertynames(df)
 
+    predefined_funs_local = predefined_funs
+
     # An array of Dicts for summary statistics
     col_stats_dicts = map(eachcol(df)) do col
         if eltype(col) >: Missing
             t = skipmissing(col)
-            d = get_stats(t, predefined_funs)
+            d = get_stats(t, predefined_funs_local)
             get_stats!(d, t, custom_funs)
         else
-            d = get_stats(col, predefined_funs)
+            d = get_stats(col, predefined_funs_local)
             get_stats!(d, col, custom_funs)
         end
 
-        if :nmissing in predefined_funs
+        if :nmissing in predefined_funs_local
             d[:nmissing] = count(ismissing, col)
         end
 
-        if :nnonmissing in predefined_funs
+        if :nnonmissing in predefined_funs_local
             d[:nnonmissing] = count(!ismissing, col)
         end
 
-        if :first in predefined_funs
+        if :first in predefined_funs_local
             d[:first] = isempty(col) ? nothing : first(col)
         end
 
-        if :last in predefined_funs
+        if :last in predefined_funs_local
             d[:last] = isempty(col) ? nothing : last(col)
         end
 
-        if :eltype in predefined_funs
+        if :eltype in predefined_funs_local
             d[:eltype] = eltype(col)
         end
 
diff --git a/src/abstractdataframe/iteration.jl b/src/abstractdataframe/iteration.jl
index 7954d011e..27034555a 100644
--- a/src/abstractdataframe/iteration.jl
+++ b/src/abstractdataframe/iteration.jl
@@ -713,11 +713,11 @@ function Base.reduce(::typeof(vcat),
 
     if source !== nothing
         len = length(dfs)
-        if source isa SymbolOrString
-            col, vals = source, 1:len
+        col, vals = if source isa SymbolOrString
+            source, 1:len
         else
             @assert source isa Pair{<:SymbolOrString,<:AbstractVector}
-            col, vals = source
+            source
         end
 
         if columnindex(res, col) > 0
diff --git a/src/abstractdataframe/subset.jl b/src/abstractdataframe/subset.jl
index 6edd21dcf..9ff5d2a3c 100644
--- a/src/abstractdataframe/subset.jl
+++ b/src/abstractdataframe/subset.jl
@@ -515,7 +515,7 @@ function subset!(gdf::GroupedDataFrame, @nospecialize(args...); skipmissing::Boo
     end
 
     # update GroupedDataFrame indices in a thread safe way
-    Threads.lock(lazy_lock) do
+    Base.@lock lazy_lock begin
         gdf.groups = newgroups
         gdf.idx = nothing
         gdf.starts = nothing
diff --git a/src/dataframe/dataframe.jl b/src/dataframe/dataframe.jl
index 2b68ea261..9bbcfbf82 100755
--- a/src/dataframe/dataframe.jl
+++ b/src/dataframe/dataframe.jl
@@ -216,15 +216,17 @@ mutable struct DataFrame <: AbstractDataFrame
         end
         len == -1 && (len = 1) # we got no vectors so make one row of scalars
 
+        len_local = len
+
         # we write into columns as we know that it is guaranteed
         # that it was freshly allocated in the outer constructor
-        if copycols && len >= 100_000 && length(columns) > 1 && Threads.nthreads() > 1
+        if copycols && len_local >= 100_000 && length(columns) > 1 && Threads.nthreads() > 1
             @sync for i in eachindex(columns)
-                @spawn columns[i] = _preprocess_column(columns[i], len, copycols)
+                @spawn columns[i] = _preprocess_column(columns[i], len_local, copycols)
             end
         else
             for i in eachindex(columns)
-                columns[i] = _preprocess_column(columns[i], len, copycols)
+                columns[i] = _preprocess_column(columns[i], len_local, copycols)
             end
         end
 
@@ -901,8 +903,9 @@ function _deleteat!_helper(df::DataFrame, drop)
         return df
     end
 
-    if any(c -> c === drop || Base.mightalias(c, drop), cols)
-        drop = copy(drop)
+    drop_local = drop
+    if any(c -> c === drop_local || Base.mightalias(c, drop_local), cols)
+        drop = copy(drop_local)
     end
 
     n = nrow(df)
diff --git a/src/groupeddataframe/complextransforms.jl b/src/groupeddataframe/complextransforms.jl
index 1a907de6f..19ddb3ac0 100644
--- a/src/groupeddataframe/complextransforms.jl
+++ b/src/groupeddataframe/complextransforms.jl
@@ -32,30 +32,32 @@ function _combine_with_first((first,)::Ref{Any},
     @assert incols isa Union{Nothing, AbstractVector, Tuple, NamedTuple}
     extrude = false
 
+    firstrow = first
     lgd = length(gd)
-    if first isa AbstractDataFrame
+    eltys = if firstrow isa AbstractDataFrame
         n = 0
-        eltys = eltype.(eachcol(first))
-    elseif first isa NamedTuple{<:Any, <:Tuple{Vararg{AbstractVector}}}
+        eltype.(eachcol(firstrow))
+    elseif firstrow isa NamedTuple{<:Any, <:Tuple{Vararg{AbstractVector}}}
         n = 0
-        eltys = map(eltype, first)
-    elseif first isa DataFrameRow
+        map(eltype, firstrow)
+    elseif firstrow isa DataFrameRow
         n = lgd
-        eltys = [eltype(parent(first)[!, i]) for i in parentcols(index(first))]
-    elseif first isa Tables.AbstractRow
+        [eltype(parent(firstrow)[!, i]) for i in parentcols(index(firstrow))]
+    elseif firstrow isa Tables.AbstractRow
         n = lgd
-        eltys = [typeof(Tables.getcolumn(first, name)) for name in Tables.columnnames(first)]
-    elseif !firstmulticol && first[1] isa Union{AbstractArray{<:Any, 0}, Ref}
+        [typeof(Tables.getcolumn(firstrow, name)) for name in Tables.columnnames(firstrow)]
+    elseif !firstmulticol && firstrow[1] isa Union{AbstractArray{<:Any, 0}, Ref}
         extrude = true
-        first = wrap_row(first[1], firstcoltype(firstmulticol))
+        wrapped_first = wrap_row(firstrow[1], firstcoltype(firstmulticol))
         n = lgd
-        eltys = (typeof(first[1]),)
+        (typeof(wrapped_first[1]),)
     else # other NamedTuple giving a single row
         n = lgd
-        eltys = map(typeof, first)
+        eltys = map(typeof, firstrow)
         if any(x -> x <: AbstractVector, eltys)
             throw(ArgumentError("mixing single values and vectors in a named tuple is not allowed"))
         end
+        eltys
     end
 
     idx = idx_agg === NOTHING_IDX_AGG ? Vector{Int}(undef, n) : idx_agg
@@ -65,19 +67,21 @@ function _combine_with_first((first,)::Ref{Any},
     sizehint!(idx, lgd)
 
     local initialcols
+    firstrow_local = extrude ? wrapped_first : firstrow
+
     let eltys=eltys, n=n # Workaround for julia#15276
-        initialcols = ntuple(i -> Tables.allocatecolumn(eltys[i], n), _ncol(first))
+        initialcols = ntuple(i -> Tables.allocatecolumn(eltys[i], n), _ncol(firstrow_local))
     end
-    targetcolnames = first isa Tables.AbstractRow ?
-                     tuple(Tables.columnnames(first)...) :
-                     tuple(propertynames(first)...)
-    if !extrude && first isa Union{AbstractDataFrame,
+    targetcolnames = firstrow_local isa Tables.AbstractRow ?
+                     tuple(Tables.columnnames(firstrow_local)...) :
+                     tuple(propertynames(firstrow_local)...)
+    if !extrude && firstrow_local isa Union{AbstractDataFrame,
                                    NamedTuple{<:Any, <:Tuple{Vararg{AbstractVector}}}}
-        outcols, finalcolnames = _combine_tables_with_first!(first, initialcols, idx, 1, 1,
+        outcols, finalcolnames = _combine_tables_with_first!(firstrow_local, initialcols, idx, 1, 1,
                                                              f, gd, incols, targetcolnames,
                                                              firstcoltype(firstmulticol))
     else
-        outcols, finalcolnames = _combine_rows_with_first!(Ref{Any}(first),
+        outcols, finalcolnames = _combine_rows_with_first!(Ref{Any}(firstrow_local),
                                                            Ref{Any}(initialcols),
                                                            Ref{Any}(f),
                                                            gd,
@@ -144,8 +148,7 @@ function _combine_rows_with_first_task!(tid::Integer,
         j = fill_row!(row, outcols, i, 1, colnames)
         if j !== nothing # Need to widen column
             # If another thread is already widening outcols, wait until it's done
-            lock(widen_type_lock)
-            try
+            Base.@lock widen_type_lock begin
                 newoutcols = outcolsref[]
                 # Workaround for julia#15276
                 newoutcols = let i=i, j=j, newoutcols=newoutcols, row=row
@@ -172,8 +175,6 @@ function _combine_rows_with_first_task!(tid::Integer,
 
                 outcolsref[] = newoutcols
                 type_widened[tid] = false
-            finally
-                unlock(widen_type_lock)
             end
             return _combine_rows_with_first_task!(tid, rowstart, rowend, i+1, newoutcols, outcolsref,
                                                   type_widened, widen_type_lock,
@@ -185,7 +186,7 @@ function _combine_rows_with_first_task!(tid::Integer,
         # This doesn't have to happen immediately (hence type_widened isn't atomic),
         # but the more we wait the more data will have to be copied
         if type_widened[tid]
-            lock(widen_type_lock) do
+            Base.@lock widen_type_lock begin
                 type_widened[tid] = false
                 newoutcols = outcolsref[]
                 for k in 1:length(outcols)
@@ -276,13 +277,14 @@ function _combine_rows_with_first!((firstrow,)::Ref{Any},
         partitions = (2:len,)
     end
     widen_type_lock = ReentrantLock()
-    outcolsref = Ref{NTuple{<:Any, AbstractVector}}(outcols)
+    outcols_local = outcols
+    outcolsref = Ref{NTuple{<:Any, AbstractVector}}(outcols_local)
     type_widened = fill(false, length(partitions))
     tasks = Vector{Task}(undef, length(partitions))
     for (tid, idx) in enumerate(partitions)
         tasks[tid] =
             @spawn_or_run_task threads _combine_rows_with_first_task!(tid, first(idx), last(idx), first(idx),
-                                                                      outcols, outcolsref,
+                                                                      outcols_local, outcolsref,
                                                                       type_widened, widen_type_lock,
                                                                       f, gd, starts, ends, incols, colnames,
                                                                       firstcoltype(firstmulticol))
@@ -378,10 +380,10 @@ function _combine_tables_with_first!(first::Union{AbstractDataFrame,
         _ncol(rows) == 0 && continue
         if isempty(colnames)
             newcolnames = tuple(propertynames(rows)...)
-            if rows isa AbstractDataFrame
-                eltys = eltype.(eachcol(rows))
+            eltys = if rows isa AbstractDataFrame
+                eltype.(eachcol(rows))
             else
-                eltys = map(eltype, rows)
+                map(eltype, rows)
             end
             initialcols = ntuple(i -> Tables.allocatecolumn(eltys[i], 0), _ncol(rows))
             return _combine_tables_with_first!(rows, initialcols, idx, i, 1,
diff --git a/src/groupeddataframe/splitapplycombine.jl b/src/groupeddataframe/splitapplycombine.jl
index 1747923df..acea4a037 100644
--- a/src/groupeddataframe/splitapplycombine.jl
+++ b/src/groupeddataframe/splitapplycombine.jl
@@ -124,7 +124,7 @@ function _combine_prepare_norm(gd::GroupedDataFrame,
         @assert length(gd_keys) > 0 || idx == gd.idx
         # in this case we are sure that the result GroupedDataFrame has the
         # same structure as the source except that grouping columns are at the start
-        return Threads.lock(gd.lazy_lock) do
+        Base.@lock gd.lazy_lock begin
             return GroupedDataFrame(newparent, copy(gd.cols), gd.groups,
                                     getfield(gd, :idx), getfield(gd, :starts),
                                     getfield(gd, :ends), gd.ngroups,
@@ -298,19 +298,19 @@ function _combine_process_proprow((cs_i,)::Ref{Any},
 
     # introduce outcol1 and outcol2 as without it outcol is boxed
     # since it is later used inside the anonymous function we return
-    if getfield(gd, :idx) === nothing
+    outcol = if getfield(gd, :idx) === nothing
         outcol1 = zeros(Float64, length(gd) + 1)
         @inbounds @simd for gix in gd.groups
             outcol1[gix + 1] += 1
         end
         popfirst!(outcol1)
         outcol1 ./= sum(outcol1)
-        outcol = outcol1
+        outcol1
     else
         outcol2 = Vector{Float64}(undef, length(gd))
         outcol2 .= gd.ends .- gd.starts .+ 1
         outcol2 ./= sum(outcol2)
-        outcol = outcol2
+        outcol2
     end
 
     return function()
@@ -384,7 +384,7 @@ function _combine_process_callable(wcs_i::Ref{Any},
 
     if !(firstres isa Union{AbstractVecOrMat, AbstractDataFrame,
                             NamedTuple{<:Any, <:Tuple{Vararg{AbstractVector}}}})
-        lock(gd.lazy_lock) do
+        Base.@lock gd.lazy_lock begin
             # if idx_agg was not computed yet it is NOTHING_IDX_AGG
             # in this case if we are not passed a vector compute it.
             if idx_agg[] === NOTHING_IDX_AGG
@@ -396,6 +396,7 @@ function _combine_process_callable(wcs_i::Ref{Any},
         end
     end
     @assert length(outcols) == length(nms)
+    idx_local = idx
     return function()
         for j in eachindex(outcols)
             outcol = outcols[j]
@@ -408,11 +409,11 @@ function _combine_process_callable(wcs_i::Ref{Any},
                     # we have seen this col but it is not allowed to replace it
                     optional || throw(ArgumentError("duplicate output column name: :$out_col_name"))
                     @assert trans_res[loc].optional && trans_res[loc].name == out_col_name
-                    trans_res[loc] = TransformationResult(idx, outcol, out_col_name, optional_i, 0)
+                    trans_res[loc] = TransformationResult(idx_local, outcol, out_col_name, optional_i, 0)
                     seen_cols[out_col_name] = (optional_i, loc)
                 end
             else
-                push!(trans_res, TransformationResult(idx, outcol, out_col_name, optional_i, 0))
+                push!(trans_res, TransformationResult(idx_local, outcol, out_col_name, optional_i, 0))
                 seen_cols[out_col_name] = (optional_i, length(trans_res))
             end
         end
@@ -443,7 +444,7 @@ function _combine_process_pair_symbol(optional_i::Bool,
     end
     # if idx_agg was not computed yet it is NOTHING_IDX_AGG
     # in this case if we are not passed a vector compute it.
-    lock(gd.lazy_lock) do
+    Base.@lock gd.lazy_lock begin
         if !(firstres isa AbstractVector) && idx_agg[] === NOTHING_IDX_AGG
             idx_agg[] = Vector{Int}(undef, length(gd))
             fillfirst!(nothing, idx_agg[], 1:length(gd.groups), gd)
@@ -463,13 +464,13 @@ function _combine_process_pair_symbol(optional_i::Bool,
     @assert length(outcols) == 1
     outcol = outcols[1]
 
-    if (source_cols isa Int ||
+    metacol = if (source_cols isa Int ||
         (source_cols isa AbstractVector{Int} && length(source_cols) == 1)) &&
        (only(source_cols) == columnindex(parent(gd), out_col_name) ||
         only(wfun) === identity || only(wfun) === copy)
-        metacol = only(source_cols)
+        only(source_cols)
     else
-        metacol = 0
+        0
     end
 
     return function()
@@ -546,7 +547,7 @@ function _combine_process_pair_astable(optional_i::Bool,
                                               wincols, threads)
         if !(firstres isa Union{AbstractVecOrMat, AbstractDataFrame,
              NamedTuple{<:Any, <:Tuple{Vararg{AbstractVector}}}})
-            lock(gd.lazy_lock) do
+            Base.@lock gd.lazy_lock begin
                 # if idx_agg was not computed yet it is nothing
                 # in this case if we are not passed a vector compute it.
                 if idx_agg[] === NOTHING_IDX_AGG
@@ -568,24 +569,27 @@ function _combine_process_pair_astable(optional_i::Bool,
             nms = out_col_name
         end
     end
+    outcols_local = outcols
+    nms_local = nms
+    idx_local = idx
     return function()
-        for j in eachindex(outcols)
-            outcol = outcols[j]
-            out_col_name = nms[j]
-            if haskey(seen_cols, out_col_name)
-                optional, loc = seen_cols[out_col_name]
+        for j in eachindex(outcols_local)
+            outcol = outcols_local[j]
+            out_col_name_j = nms_local[j]
+            if haskey(seen_cols, out_col_name_j)
+                optional, loc = seen_cols[out_col_name_j]
                 # if column was seen and it is optional now ignore it
                 if !optional_i
-                    optional, loc = seen_cols[out_col_name]
+                    optional, loc = seen_cols[out_col_name_j]
                     # we have seen this col but it is not allowed to replace it
-                    optional || throw(ArgumentError("duplicate output column name: :$out_col_name"))
-                    @assert trans_res[loc].optional && trans_res[loc].name == out_col_name
-                    trans_res[loc] = TransformationResult(idx, outcol, out_col_name, optional_i, 0)
-                    seen_cols[out_col_name] = (optional_i, loc)
+                    optional || throw(ArgumentError("duplicate output column name: :$out_col_name_j"))
+                    @assert trans_res[loc].optional && trans_res[loc].name == out_col_name_j
+                    trans_res[loc] = TransformationResult(idx_local, outcol, out_col_name_j, optional_i, 0)
+                    seen_cols[out_col_name_j] = (optional_i, loc)
                 end
             else
-                push!(trans_res, TransformationResult(idx, outcol, out_col_name, optional_i, 0))
-                seen_cols[out_col_name] = (optional_i, length(trans_res))
+                push!(trans_res, TransformationResult(idx_local, outcol, out_col_name_j, optional_i, 0))
+                seen_cols[out_col_name_j] = (optional_i, length(trans_res))
             end
         end
     end
@@ -675,15 +679,15 @@ function _combine(gd::GroupedDataFrame,
         return Int[], DataFrame(), Int[]
     end
 
-    if keeprows
+    idx_keeprows = if keeprows
         if nrow(parent(gd)) > 0 && minimum(gd.groups) == 0
             throw(ArgumentError("select and transform do not support " *
                                 "`GroupedDataFrame`s from which some groups have "*
                                 "been dropped (including skipmissing=true)"))
         end
-        idx_keeprows = prepare_idx_keeprows(gd.idx, gd.starts, gd.ends, nrow(parent(gd)))
+        prepare_idx_keeprows(gd.idx, gd.starts, gd.ends, nrow(parent(gd)))
     else
-        idx_keeprows = Int[]
+        Int[]
     end
 
     idx_agg = Ref(NOTHING_IDX_AGG)
diff --git a/src/groupeddataframe/utils.jl b/src/groupeddataframe/utils.jl
index 9d882fcf9..652d87acb 100644
--- a/src/groupeddataframe/utils.jl
+++ b/src/groupeddataframe/utils.jl
@@ -261,6 +261,24 @@ function row_group_slots!(cols::Tuple{Vararg{AbstractVector}},
     return ngroups, rhashes, gslots, false
 end
 
+function _reduce_or!(x::AbstractVector{Vector{Bool}})
+    len = length(x)
+    if len < 2
+        return
+    elseif len == 2
+        x[1] .|= x[2]
+    else
+        xl = view(x, 1:len ÷ 2)
+        xr = view(x, len ÷ 2 + 1:len)
+        t1 = @spawn _reduce_or!(xl)
+        t2 = @spawn _reduce_or!(xr)
+        fetch(t1)
+        fetch(t2)
+        xl[1] .|= xr[1]
+    end
+    return
+end
+
 # Optimized method for arrays for which DataAPI.refpool is defined and returns an AbstractVector
 function row_group_slots!(cols::NTuple{N, AbstractVector},
                           refpools::NTuple{N, AbstractVector},
@@ -424,25 +442,7 @@ function row_group_slots!(cols::NTuple{N, AbstractVector},
         end
     end
 
-    function reduce_or!(x::AbstractVector{Vector{Bool}})
-        len = length(x)
-        if len < 2
-            return
-        elseif len == 2
-            x[1] .|= x[2]
-        else
-            xl = view(x, 1:len ÷ 2)
-            xr = view(x, len ÷ 2 + 1:len)
-            t1 = @spawn reduce_or!(xl)
-            t2 = @spawn reduce_or!(xr)
-            fetch(t1)
-            fetch(t2)
-            xl[1] .|= xr[1]
-        end
-        return
-    end
-
-    reduce_or!(seen_vec)
+    _reduce_or!(seen_vec)
 
     # If some groups are unused, compress group indices to drop them
     # sum(seen) is faster than all(seen) when not short-circuiting,
diff --git a/src/join/composer.jl b/src/join/composer.jl
index 3cd4e90b3..f4ba5d853 100644
--- a/src/join/composer.jl
+++ b/src/join/composer.jl
@@ -256,14 +256,17 @@ function compose_inner_table(joiner::DataFrameJoiner,
         right_ixs = right_ixs[csp_r]
     end
 
-    if Threads.nthreads() > 1 && length(left_ixs) >= 100_000
-        dfl_task = @spawn joiner.dfl[left_ixs, :]
-        dfr_noon_task = @spawn joiner.dfr[right_ixs, Not(joiner.right_on)]
+    left_ixs_final = left_ixs
+    right_ixs_final = right_ixs
+
+    if Threads.nthreads() > 1 && length(left_ixs_final) >= 100_000
+        dfl_task = @spawn joiner.dfl[left_ixs_final, :]
+        dfr_noon_task = @spawn joiner.dfr[right_ixs_final, Not(joiner.right_on)]
         dfl = fetch(dfl_task)
         dfr_noon = fetch(dfr_noon_task)
     else
-        dfl = joiner.dfl[left_ixs, :]
-        dfr_noon = joiner.dfr[right_ixs, Not(joiner.right_on)]
+        dfl = joiner.dfl[left_ixs_final, :]
+        dfr_noon = joiner.dfr[right_ixs_final, Not(joiner.right_on)]
     end
 
     ncleft = ncol(dfl)