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It's not the allocation itself that's faster, it's faster because it avoids the overhead of crossing the C++ -> JS barrier

My guess is that it was previously implemented in nodejs C code instead of V8, and they switched the nodejs buffers to take advantage of V8 ArrayBuffers.

Malloc in C/C++ is not particularly fast.

Java for example does object instantiation much faster.

It's also common to replace malloc with an in house implementation for performance sensitive code.