Be slow to swap out fat tires
Do your homework before you swap out the knobbies. Last Spring, I switched into skinny road slick type 26" tires, but I lost top speed and I lost circumference: Not happy. This Spring, I switched into 2.1 fatter pseudo-slicks and am much happier. They are the Continental Town+Country tires which have an alternating side tread but keep the same size/dimensions as the knobs.
True, the heavy mountain bike with fat tires make it more challenging on the climbs, but I enjoy the challenge. I rode a 100mi century with roughly 8,300ft of climb last month and did it in a very respectable 6.5 hours; respectable given the elevation and the bike limitations. Oh, it has fenders, rear rack and travel bag too.
All that to say to do your homework before you swap into some skinny slicks.