A load on either end of the bike is going to increase drag.
For practical purposes, on a normal bike, that's not true.
Carrying anything on a rack OR on your back will make it a little more difficult to ride up hills.
On flat ground, 20lbs of stuff on a rack or on your back will probably slow you down a few seconds for every hour of riding.
P.S.- it's true, if you want to be as safe as possible when braking hard, put stuff on the back rather than the front.