20 hours of work is pretty extreme and I suspect cuts down on your mental ability to do research (you have to take classes, too). I suspect the department usually has to say it is a 20 hr/wk position (probably so student gets free tuition), but the actual work is often less -- my friend who went to U of Maryland for PhD in CS had a similar experience to mine -- 7-10 hours a week of actual work at most.

This is just a couple of data points, though. Maybe we were just lucky.