So we aren't using mmap (but we are using WAL), so for one thing there's a lot of buffer and cache copy work going on - I'd expect it's the old classics from string.h. WASM is a high level VM though so I'd expect (provided you pass the relevant flags) it should be using e.g. the VMs memory.copy instruction which will bottom out in e.g. v8s memcpy.

we also don't use a common driver, ours is here https://github.com/tailscale/sqlite

sqlite in general should be mostly optimized for the targets it's often built with, so for example it'll expect everything in string.h to be insanely well optimized, but it should also expect malloc to be atrocious and so it'll manage it's own pools.