Does it feel like it took surprisingly long to port it?

I expect Photoshop to contain many highly optimized function routines for photo processing. These routines are optimized by hand and usually use architecture-specific features such as SSE/AVX which is not portable.

Unoptimized (more portable) versions of theses routines exists, but I suspect they will perform well even with M1.

I don’t have hands-on experience, but somewhere on HN I saw this: https://github.com/simd-everywhere/simde If starting a new cross-platform project today, I would try that library first, before doing the usual intrinsics.