I had a mtb that I equipped with slicks but it was so much heavier and slower than my road bike that I stopped riding it altogether (I stopped commuting on it about 8 years ago in favor of an old early 90s road bike I had after trying to make the mtb work as a commuter).

I actually sold my mtb last summer and don't miss it at all. Who has time to pack your bike up on the car, drive an hour to someplace where you can ride it off road, then spend a couple hours riding, only to drive an hour back home. Not me...By by worthless hunk of metal. Hello road bikes!!!