Spent $180 on a rotary chimney brush kit that just bounced off my flue tiles

Bought one of those rotary brush kits online for $180 because a guy at the supply house in Boise swore it would cut my sweep time in half on creosote. First run up a glazed flue and the thing just skipped off the tile like a rock on a pond, barely touched the hard deposits. Turns out my drill was too fast and I was pushing way too hard, so I chewed up two of the nylon whips in one job. Had to fall back on my old steel brush and rods for the rest of the day and still finished the stack, which honestly stung more than the money I lost. Funny part is I finally got the hang of the thing last week on a soft soot liner and it worked great, so the kit wasn't bad, I was. Anyone here running a rotary brush, what rpm and feed speed are you using so I don't toast another set of whips?