I know they run slower as the bag fills up, so maybe get the bag dirty before you start. Kinda seems illogical, seeing as the bag is supposed to help it clean, not get it dirty. But hey, why not?
Obviously you'll have to experiment with how much and what not. I'd use DE to "pre-dirty" it. Or maybe just stuff it full of nylons, socks, old briefs, boxers, etc..
I don't know if it'll slow it down enough, but I do know that is a real problem with many an automatic pool cleaner, they go too fast to keep from stirring up the fine stuff. It's not about speed, it's about accuracy and detail with these guys.
The only other thing I could think of is to modify your power supply so you have a variac type resistor on the DC side, this way as you turn down the voltage, it will slow down the rover. Since you're just grabbing fine dust and the wheels run off the vacuum motor, being that it doesn't take much suction to pull up fine dust, you should be good.
But you'd have to be good with electronics or know someone that is to perform this. Do not attempt it yourself if you aren't.
Or buy a variable output bench power supply off Fleabay.
