He's not the fastest, he just stands well back, has the long limbs, and slides a lot and controls the balls well on defensive shots.
Once Ferrer did genuinely get the ball out wide and Monfils had to make up a lot of ground, Monfils was nowhere in sight usually.
the best movers in the game are Fed, Djokovic, and Nadal. the fastest (one with the most speed) might be Blake.
that said, Monfils is a great athlete. i would say that Nadal may be a little faster than Monfils.