It certainly can add up.Quicker, not masses quicker but a couple of mph and that can certainly add-up over a longer commute.
If it's for mainly road use then a thinner tyre will be beneficial as they will roll easier and faster. 26x1.5 or 26x1.75 are most likely to fit your rims.Im was thinking for doing the same, but dose the tyre width make much difference? ive got a mountain bike with think tyres on them, so do i need to get the same or should i get thinner ones? i think the ones ive got now are 26 x 2.35