You'll see similar performance variations of the same C code across different compilers, different CPU architectures, on the same CPU when the OS decides to switch between performance- or efficiency-cores, or even just slightly different compiler optimization options (e.g.: real-world performance of native code already varies for many reasons, and WASM is roughly in that same ballpark).
FWIW, in my home computer emulators written in vanilla C (e.g. no language extensions like SIMD intrinsics) the performance difference on an M1 Mac between the native version and WASM version running in Chrome (e.g. https://floooh.github.io/tiny8bit/c64.html) is indeed within 10%.
This is mostly straightforward integer bit-twiddling code, other types of code may behave differently (one should expect up to 2x slower as worst case).
You'll see similar performance variations of the same C code across different compilers, different CPU architectures, on the same CPU when the OS decides to switch between performance- or efficiency-cores, or even just slightly different compiler optimization options (e.g.: real-world performance of native code already varies for many reasons, and WASM is roughly in that same ballpark).
FWIW, in my home computer emulators written in vanilla C (e.g. no language extensions like SIMD intrinsics) the performance difference on an M1 Mac between the native version and WASM version running in Chrome (e.g. https://floooh.github.io/tiny8bit/c64.html) is indeed within 10%.
This is mostly straightforward integer bit-twiddling code, other types of code may behave differently (one should expect up to 2x slower as worst case).