I haven't honestly dug around to figure out if there's a hardware reason for it, but prompt processing has always been a lot slower for me on macs in general. I mostly use MLX on my 24GB M4 Pro though, so I will pull llama.cpp on it as well to see what the prefill is like.

I've gotten around 16 t/s gen with 4bit and mxfp4 on that model for generation. The 3090 I mentioned has a little over 900 gb/s, while those macs i think are around 270 GB/s. If my understanding is correct, macs do utilize the bandwidth better in this case, but it still doesn't make up the difference (on the 3090 it's around 30-35 t/s depending on size of ctx).

Also, do run a quick experiment removing the cache quants if you want to tinker with it a bit more, iirc KV quant does add a small overhead during prefill.

I would be very interested to know your prefill and generation numbers.