Originally Posted by Retro Grouch

"Feels" slower or are you measuring it some way?

Realistically, once you reach around 15 mph air drag surpasses everything else that's holding you back combined. As you speed upward from 15 mph, that percentage continues to increase. I don't doubt that a decently maintained drivetrain will have a benefit but, if your objective is to ride faster, your effort will always be better spent by improving your aerodynamic positioning on your bike.