I love Witch, but after upgrading to Mavericks I found that it was weirdly sluggish and tended to overshoot what I was aiming at. It also doesn't function with a lot of fullscreen apps (primarily games) and iBooks does ridiculous stuff with its windows that no app switcher seems to know how to work around. All that said, I'm still glad I bought it and I end up switching back and forth to the default app switcher because it's so much better in every way but those bugs. I'm sure the iBooks thing will get resolved and I think the sluggishness thing is eventually going to get cleared up in patch.

For what it's worth, I haven't had that slowdown experience neither on Mavericks nor on Yosemite. Given I for example don't use full screen at all.