I just gave it a spin, it took 1 min 52 sec for a 50 steps image and that is ~2.2s/step. It seems faster than my original installation(which might also have improved speed as it was at very beta stage when I tried it) but definitely not 20 seconds for 50 steps image at 512x512 resolution.

Maybe they use lower parameters.

edit:

50 steps at 256x256 resolution took 55 seconds.

50 steps at 768x768 resolution took 8 min, exactly.

PS: my Macbook Air is modified with thermal pads, it takes a bit longer to start throttling than usual. Either way, it's very dependent on the ambient temperature.