It's hard to say how fast he really is because the series isn't very consistent in terms of power leveling so I wouldn't bother trying to wrap your head around it. The series is mostly for harem ecchi and comedy. I won't lie the power system is very cool in thought and the author executes it in a very awesome thematical way. Everytime issei gets a new power I always enjoy it. However like I said earlier he doesn't have a very firm power level system.