My first serve probably tops out at 90mph, but many times it is probably in the 80s. Nevertheless it can still ace people 4.0 or lower occasionally (even a 4.5 on a rare occasion). Even though it is the weakest part of my game, I usually have a high first serve percentage and I place it well, so it does the job mostly, it is just not dominating. 90mph is still a fast serve, I think most people grossly overestimate the speed of their serves/hits if they don't have an accurate and properly used radar gun.
A 110 mph serve that you get in 20% of the time is effectively useless, it will lose matches for you, and will hinder your progress on your serve if you keep trying it. You have to build up to that sort of serve.