I turn at a lot higher speeds than I used to. It is a comfort thing. Whatever speed you feel comfortable at is the right one. I do find it easier to turn at high speeds, and I do seem to get a cleaner cut. For bowls, I turn between 300 or so up to 1500, depending on size, how well it is balanced, and if the lathe is shaking. I would turn smaller bowls (8 inch or so and smaller) at higher rpm, but I would have to change the belt, and that would end up being for the last core or 2 that I do, then I would have to change back..... Too much extra work. Spindles, I turn up to 3000 rpm, and some times I could see that being too slow. Some of the Myrtlewood turners on the Oregon coast turn Myrtle bowls at 4000rpm. The thought of that maked all my sphyncter muscles pucker. A friend of mine has one of the Legacy ornamental lathes, which has a top speed of 200 rpm. When he comes over to core something, he turns at that speed on my lathe. It drives me nuts, but that is what works for him.
One theory of mine on the cleaner cuts at higher rpm(besides like John said about body movement being easier) is that I get more cuts per inch. By that I mean if you are moving at the same rate/pressure on the tool as when you turn slower, you will get more cuts per inch of wood. The cuts are closer, and there are less lines in the wood. The cuts also are a bit finer and lighter so less tearing.
robo hippy