When I transitioned my 17 mile commute to work from a MTB with knobbys to slicks I gained about 2-3 MPH on average. Then, when I switched to riding a light carbon RB I gained another 1-2 MPH. So for me at least the biggest benefit was ditching the knobbys (~20% gain), compared to the difference of RB over MTB with slicks (~10%). Obviously there are a lot of variables, but this was my experience.