They're not all full time I don't think, but a lot of the ones with really long comics have assistants to help. As others have said they're really fast and talented. They're professionals afterall.
But I've also noticed these 3 things which I think help some(not all) be faster:
I think they are at the point where they know what battles to chose. I mean if you really analyze every panel, yes you'll see some occasional funny looking hands, or proportionally off poses, etc. But they know how to draw a lot of your attention away from those details in those panels. Meaning when they do make mistakes or have weaker art, they do it in a manner where it doesn't effect the quality of the panel because its not something you're really looking at or studying when you read the comic as a whole.
Many of them use copy/paste. Oh your character didn't move much but changed expressions? Copy paste the body/head zoom in alittle to the face, and change the facial expression, boom new panel with a lot less effort. Or say you're making a background with lots of trees? draw like 3 different trees and copy paste them to make your forest (or use a tree brush if you have the right program- CSP Assets as a whole saves me SOOOO much time)
Many have 3d backgrounds, if you know what you're doing with the 3d backgrounds, you can make those faster than drawing them. And you can manipulate the angles how you like so you can have a scene in a complicated location but only make the model for that location once. And some 3d models are already made, there's a 3d model of a classroom in CSP, I've used it before cause you can just change the angle how you like and then never draw all those annoying desks and chairs in proper perspective.