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 <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.
I've made this change myself. You won't notice a speed improvement, but there will be a very slight effort reduction that might lead to a tiny bit more speed. You'll notice that you don't slow down as fast when coasting, meaning it takes a tiny bit less power to maintain speed when pedalling. But not enough to go noticably faster.
The biggest improvement will be the noise reduction :-) And fast cornering (on the road) will feel more secure.
I think you will notice a difference, but if/when you go back to knobbly mountain bike tyres, you will notice a huge difference. It will be much more noisy, going round corners at any speed will feel strange and insecure and you will not coast anything like as far when you stop pedalling. From all this, I think you can conclude that the effort for the same speed is reduced with slicks. You will also start to think that mountain bikes with 2.1 inch tyres look like they have doughnuts for wheels.
You will also probably notice that your tyres last longer too. Long distance commuting on knobbly types wears out the central knobs, leaving them unsuitable for much at all. If you commute in the wet, the knobs also throw up more water from the sides that lands on you.
I have used unbranded back street bike shop 26x1 and Specialized 26x1.5 road tyres (with tread, not slicks) on my mountain bike for all weather commuting and both lasted for ages.
Make sure you pump the tyres up to their recommended pressure.