I've tried llama.cpp's new --moe-cache-mib flag and it lowers performance to about a third of what it is without the flag.

And no wonder really - PCIe 4.0 x16 has a one direction bandwidth of around 31.5GB/s - much slower than system RAM - in my case about a third the speed and I think it's no coincidence that I'm getting this exact rate of slowdown.