I generally set mine on a low to medium speed. I listen to it and if it seems to be working too hard, I "throttle it back" some. If it's going too slowly and doesn't seem to be straining, I crank it up a bit.
Although, I confess I quit using mine for tomatoes...the holes are so darned small you end up with hardly any pulp and lots of liquid. For juice that's great, but for canning tomatoes or sauce, not so much. We got a Roma Food Mill from Lehman's a couple months ago and ordered the salsa kit...the holes are much bigger and I like it a LOT! I strain after running the 'maters through, and use the stuff IN the strainer for sauce and the stuff that runs through for juice. Then, I run all the peels and such through the Champion juicer. By the time I'm done, there's not much left other than practically dry leavings.
We did 3 boxes of romas yesterday in this manner, and also juiced some beets, onions, celery, parsley, basil, carrots and zucchini to add to the juice for a vegetable cocktail sort of thing. Took DH and me about 4 hours to process it; it was awesome!