It does depend a lot on the bike. A lot of commuters like panniers and wide tires with flat protection. A bike like that might have a typical 10-15mph average speed.

I get a 20% improvement switching to high performance skinny tires (i.e. 25mm gatorskins).

As for racing, I can add on another 20-30% improvement for riding in a pack (which reduces aero drag when drafting) and maybe a 50% improvement from the adrenaline used in an actual race.

But for commuting, I purposely go out of my way to slow down, ride through parks and green spaces, and just enjoy parts of the commute and being outside in the morning or evening.