Ghost writers, usually, yeah. But the funny thing is, it's usually just one ghost writer! Me!
Seriously, I've filled a few hubpages accounts for other people before deciding to fill my own (and I've stopped ghosting for hubbers since I found out how neat the place is, and how unfortunately tainted it is by the quick-buck people I used to write for).
I've definitely slowed my pace down in writing my own stuff, but I'd say that if you have a lot on your mind that day, you might be able to easily squeeze off anywhere from five to ten before hitting the hay.
Of course, doing that on a daily basis definitely means that they're either hiring a ghost who just doesn't care (like me) or they just personally don't care about the quality (UNlike me).
Honestly, experience as a ghost writer has been a big help. You HAVE to write ten, twenty articles a day if you want to pay the rent. But quality suffers if you're writing those pages because you have to, not because you want to.