I've tried Clip Studio, but only the trial version and while it work nicer than Photoshop, I didn't want to spend money on it because I seldom draw comics. The way I see it, Clip Studio works perfectly for comic/ manga enthusiasts or creators, but Krita is splendid for illustrations. But we all have one software that we must be really good with.

There was a solution to lagging Krita, which was to disable OpenGL. I never had any problem with Krita. It used to be hard when it first came out, but nowadays it feels natural because the interface is much better.

I seldom draw portraits because it's not really my cup of tea; I've got several friends and acquaintances whose works consists mainly of portraits though. It's a good money maker though IMO because from where I come from, people mainly ask for their faces to be drawn heheh.