My personal experience after switching to Colemak is mostly neutral. Speed is about the same after some training, around 70 WPM. Comfort, maybe improved a bit, but no life changing.

Some people claim that they went from 60 WPM on Qwerty to over 100 WPM on some other newly designed layout, but my experience is clear: if you do it for the speed you will be disappointed.

I'd guess the speed improvement in those cases likely came from learning a better technique, like touch typing and using more of your fingers. Afaik a lot, if not most, of the fastest typists are still on qwerty.