Will a change from 26x2.10 to 26x1.95 tires improve speed? If I change my 26x2.10 MTB knobbies by 26x1.95 slick tires, will I feel any difference on my speed on paved roads? or is it going to be the same? I am kind of confused about how much I should change the width of my tires to notice an improvement in my speed. The reason: I will be using my MTB to commute.
If you run your knobbies at typical MTB pressures, and you run your slicks at higher pressures, you should experience less rolling resistance, which could translate to higher speed.

I don't know what tires you're talking about exactly, so it's hard to say how much of a difference it will make. I run some pretty aggressive knobbies on my MTB (Panaracer Fire XC Pros) and with &lt;35 PSI in them, that makes for some noticeable rolling resistance. On my vintage no-suspension MTB, I run slicks with 60-65 PSI in them, and those are halfway to being road-bike-like. Even with the MTB gearing, I can hold 15-16 mph on that bike for a decent amount of time.

You did not indicate what kind of suspension your MTB has. With any suspension, you're probably going to get a little pedal bob, which will rob you of some energy. That, combined with the upright riding position, will hold you back a little. But the engine (your strength) is more of a factor than tires, suspension, and bike weight.

Once upon a time a friend of mine-- road bike rider, likes Italian bikes, pretty fit at the time-- used to regularly get passed on a local muni trail by this little guy on a vintage mountain bike who was dressed in street clothes and riding to/from work. These roadies would be doing a paceline at a good speed, and the little guy on the mountain bike would just roll right past them, comfortably holding 2-3 mph faster. I wasn't there, so I can't tell you if the mountain bike guy had modified his bike or what. But he used to say "good morning" or something similar, every time he passed them, with a big smile.