Road tires really don't go above 28mm, so what you are really looking at is slick MTB tires, not what would be considered a road tire.

Going narrower will mean you need to run higher pressure, it's a trade off between comfort and speed (up to a point).

For weight, without knowing the specific tires, it's pretty irrelevant, as you could have a heavy thin wire bead slick with high roll resistance or a high volume light weight Kevlar high TPI tire with low roll resistance, what you will find is to get light weight, you will pay for it.

For running slicks, years ago I was using Vredestein s-licks 1.3 tires (these are long since discontinued) am now riding the same bike on 1.9 semi-slicks, and they are much more comfortable than the 1.3's and the loss of speed is minimal.