hot air temperature affect outboard speed

does the 90 degree temp and high humidity affect the outboard engine speed verses in the spring of the year when the air temp is in the 60s? my fishing partner tells me his merc 175 will power his boat around 65 mph in the cooler weather but this time of year he is lucky to get 60 mph out of it. or is their a chance he has a problem with this engine.

YES!!!!!!!!

yes it will knock off a few hundred rpms,especially if it propped a little to much or you pack a lot of weight you'll really notice it.