[robko]

You are correct. The code is using an inefficient cache access pattern, so most of the time is spent waiting.

You probably won't get 100x faster without SIMD, but 10x is certainly doable. Unfortunately, SIMD.js support has been removed from Chrome and Firefox a while ago, even though it is not available in wasm to this day.

[kllrnohj]

How would SIMD do anything to address the problem's fundamental anti-cache-friendly access patterns? You'd need to restructure the problem to be cache-friendly, but SIMD won't really be relevant to that.

[robko]

You can use both at once. Usually, you'd have something like 64x64 tiles in cache and use 4x4 or 8x8 tiles for SIMD.