As you've been told, it depends on the rim width.
Also, tyre speed isn't related as strongly to width as people think - or even in the way that most people think. Read the tech docs on Conti and Schwlabe's sites.
The best MTB slick at the moment is probably the 1.5 Rubino Pro. It's made of real racing tyre rubber and has much more wet weather grip than the Fatboy, which is about as fast. Conti Sport Contacts are worth considering if you can't get the Rubinos.
Other thing that you probably haven't been told: inner tube quality and talcing have an effect on rolling resistance also.