I had a fun 2 years where I basically had 800 critical lines of math code that had to be on every platform, one version in the native language of the platform, and one version as performance optimal as possible.

I was absolutely stunned to find that on web WASM ~= JS which altogether weren't far off from a naive C++ stdlib port. I had to make custom data structures instead of using stdlib ones to really get performance significantly different . IIRC my most optimized C++ converted to WASM was about 70% the speed of JS.