I use iStat Menus for this. An excellent series of tools that lets you monitor just about everything that can be monitored on your Mac.

App Tamer will help you keep unruly apps under control, forcing them to run on efficiency cores instead of performance cores, slowing them down if they eat up more than X% of your CPU, stopping them entirely if they are in the background, and so much more.

The combo of iStat Menus and App Tamer is what makes it tolerable for me to have a MacBook Air as the primary laptop I use these days.

I love the menubar UI idea!