In general, they have a weird kit, because he generates TM, so even making him slow doesnt do much in the way of hurting him, but his TM generation hurts him because you want him to wait while the enemy moves to build up charge.
The way his kit is played out low speed doesnt hurt him too much, and it helps his damage out put.
I cant see any real reason for him to be super fast that is tremendously helpful.
I'm sure there is a balance somewhere between more moves(higher speed) at "lower damage" and slow as a rock with "higher damage" (potential). IMO, I'm going to try slow and go up as I practice more to see what happens.