Why would someone driving a Porche BMW Mercedes or any other fast sportcar decide to go 15 miles below the speed limit when they most certainly have the car to go faster?

The Real reason is they don't want everybody in the world thinking that they have to show off in front of some idiot in a pinto thinking that their car belongs on fast and the furious. It is a personal preference showing off does not happen in 5 o'clock traffic catch one at night you might see its true power.

Answer

This probably has something to do with personal choice. A more comforting answer might be that they can't really afford the car and are afraid to get into an accident, or realize that they're a higher-profile target for police to give speeding tickets to.