Don't patronize us experts, asshole!
From what I've read, it makes little difference. There is some opinion that lower speed motors work better as idlers though I've no idea why that might be so. They're sure as hell less noisy.
I still don't fully understand how an RPC really works. Fitch admits that he doesn't either though I've no doubt that he could if he wanted to bad enough. We found some academic papers that purported to analyze them but the math swamp was way thicker than we've had the ambition to journey thru so far.
Jerry did some very interesting research with good instrumentation and a shop-made eddy-current brake that worked really well, had some findings contrary to "informed opinion". Oh well! His apparati are now dismantled and up for grabs for the quick, dumpster-destined otherwise.