Originally Posted by
Skipjacks
Knobby MTB tires make for crappy commuting on pavement tires. They slow you down and just aren't well suited to pavement.
+1
My 1997 Nishiki Blazer unsuspended mountain bike was my main commuter until 2015, it is still my winter commuter and utility bike. And I still ride it for variety.
Besides knobby tires, the other challenge was the low top gearing. I fixed that by putting on the biggest front chain ring that would physically fit. Now it has a tremendous gear range, and is fast; heavy, but fast.