Windows' malloc has awful performance. mimalloc is orders of magnitude faster, even faster than DeletedBufferChain. Therefore, only enable USE_DELETED_CHAIN on Windows when building without mimalloc.
On Linux, mimalloc doesn't appear to be measurably faster than glibc's own allocator. Both are marginally than DeletedBufferChain, though, and substantially faster in the multi-threaded case, so USE_DELETED_CHAIN is disabled there in all cases.
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