Studs...Drag? Lots. My 20 min commute goes to 30 min in winter with studs. Some of that may be thicker clothing and adjusting to the colder weather in general, but I think a lot of it are the drag associated with the studs.

(BTW, I used Nokian for 3 winters with no complaints. Last year I gave the nashbar studded tires a go and they held up very well. If they make it through this winter I think I'll switch, since $40 per tire is WAY better than $80 per tire, even if its only 2/3 the life.)