The thing to understand is that the 8GB M1 has 8GB. When I run txt2img.py, my Activity Monitor shows a Python process with 9.42GB of memory, and the "Memory Pressure" graph spends time in the red zone as the machine is swapping. While the 16GB M1 Pro immediately shows PLMS Sampler progress, and consistently spends around 3 seconds per iteration (e.g. "3.29s/it" and "2.97s/it"), the 8GB M1 takes several minutes before it jumps from 0% to 2% progress, and it accurately reports "326.24s/it"
So yes, whether it's M1 vs M1 Pro, or 8GB vs 16GB, it really is that stark a difference.
Update: after the second iteration it is 208.44s/it, so it is speeding up. It should drop to less than 120s/it before it finishes, if it runs as quickly as my previous install. And yes, 186.04s/it after the third iteration, and 159.22s/it after the fourth.
Looks like RAM drastically affects the speed.
It looks like memory is super-important for this (which isn't all that surprising, really...).
I had a YouTube video playing while I kicked off the exact command in the install docs, and got: 16.84s user 99.43s system 61% cpu 3:08.51 total
Next attempt, python aborted 78 seconds in! Weird.
Next attempt, with YouTube paused: 16.31s user 95.48s system 65% cpu 2:49.45 total
So around three minutes, I'd say.
On a first-gen M1 Mac mini with 8GB RAM, it takes 70-90 minutes for each image.
Still feels like magic, but old-school magic.