This is a bit of a drastic example but I have 35 lbs steel commuter bike with rack, bag, etc that I rode all the time until earlier this year when I got a 17 lbs bike (both bikes listed in signature). I have a 16 mile loop that I often do and it has a few hills, or at least what we call hills in FL.

With the heavier bike, my best was a 17.3 mph average (did that twice). On my first ride with the new bike, I averaged 18.1 mph. On the second ride with the new bike I averaged 18.7 mph.