I've installed rooms manager seems like an okay app but I'm not convinced it's worth keeping installed. My question is will this app slow down my hub anymore or any less then just creating my own rules?
https://github.com/adey/bangali/blob/master/README.md
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).
I used it for close to a year. But only as a button input, I used the button controller app for the rule execution. I didn't notice any slow downs though. I've now moved all but one to RM3 since it can handle more complex situations now. For a beginner it's a great way to accomplish some complex situations.
I actually uninstalled Rm and noticed the speed of the hub go back to normal it just seemed a bit sluggish when i had RM installed. I'm switching over from vera plus so with with RM3 it's very similar, the other rule machine apps I thought were horrible! RM3 is a step in the right direction. Vera is not at all as good as hubitat but the way you make up rules is way better compared to Hubitat's old Rule machines, down right caveman style. I was like where's the if & or options? RM3 could still use some help though if you happen to select something wrong some of the conditions can't be removed and you have to delete the rule??? Maybe I'm just missing something but I've had to actually start the rule over from scratch.
