Different performance between reduce(map()) and mapreduce()

General Usage Performance
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reduce has a specialization for vcat where it preallocates an output array first and then loops over to fill it. mapreduce isn’t so clever here.

See Optimized methods for `mapreduce(f, vcat, xs)` and `mapreduce(f, hcat, xs)` · Issue #31137 · JuliaLang/julia · GitHub

It is a bit unfortunate that specializations for vcat/hcat are not mentioned in the documentation AFAICT.

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