Passing the guard can be a meticulous process, especially without a gi. No gi is much more slippery, you lose the ability to slow down the pace of a match and the young guns, athletes, and flexible guys thrive.
With out the grips people find a way to squeeze their way out of any position, legs come out of no where to retain guard, and the pace of the rolls seems to be much higher.
Fabiano has an answer for all of this. He has created a methodical approach to passing the guard without the gi that still allows him to close the distance, stay tight, and move at a slower pace.
No need to over exert, use cardio, or strength. Just get the right grips, and distribute your weight properly so that your opponent can't move. Make yourself feel like a boulder when passing the guard and slowly grind your opponent out until they are begging you to pass.