It currently uses only the CPU via ARM NEON intrinsics - no GPU, no ANE and no Apple Accelerate.
Plan is in the future to utilize respective SIMD intrinsics for other architectures (AVX, WASM SIMD, etc) and also add other more accurate quantization approaches. It's actually not a lot of work and I have most of the stuff ready, so hopefully soon!
Edit: AVX2 support has just been added