Tumbler speed settings?

I have a Nat Geo-equivalent tumbler (Dr. Cool, if you're interested, lol) and it's got the standard days/speed settings.

So far I've only had it on Speed 1, but I'm wondering if anyone here has preferred speed settings for different recipes/ stages, and how that might affect results. I've been tumbling Mohs hardness 6-7 rocks only so far, but after this batch I'm going to do some 4-5 hardness serpentine, and I'm preparing to run that set entirely differently.

Any and all thoughts appreciated! Cheers