I don't think anyone has reported hub slowdowns as a result of installing this app. It's always a possibility with poorly written custom code, but there are probably enough people using it that someone would have noticed if it were poorly written by now. 
But if you're asking how responsiveness of automations created using Rule Machine vs. this app will compare, the difference is likely to be milliseconds at most, and I don't even have a guess as to which might come out on top (but if you made me pick, I bet it might be bangali's Rooms Manager since Rule Machine is quite a massive app and you're likely only using a fraction of its capabilities, but I'm sure it's very close either way). If Rule Machine is capable of doing everything you want, however, without creating lots of rules and conditions, it's likely Rooms Mananger is overkill and Rule Machine might be simpler to set up (I've also been told my Lights on Motion Plus app is like a "light" version of Rooms Manager, but I was never able to figure out Rooms Manager enough to dig into its functionality, so I can't actually make any comparisons).
