What's difficult with font SDFs on the CPU? The bezier paths?
I made myself a CPU SDF library last weekend, primarily for fast shadow textures. It was fun, and I was surprised how well most basic SDFs run with SIMD. Except yeah Beziers didn't fair well. Fonts seem much harder.
SIMD was easy, just asked Claude to convert my scalar Nim code to Neon SIMD version and then to an sse2 version. Most SDFs and gaussian shadowing got 4x speedup on my macbook m3. It's a bit surprising the author has so much trouble in Rust. Perhaps fp16 issues?
I haven't looked at this recently but from what I remember rendering from SDF textures instead from simple alpha textures was 3-4 times slower, including optimizations where fully outside and inside areas bypass the per pixel square root. Of course SIMD is a must, or at least the use _mm_rsqrt_ss.