"System activity" isn't a valuable user metric because not all CPU %s are equal and CPU % isn't a consumable resource. Fans, battery life, case temperature, some others are.
System activity can certainly cause problems like paging out all the file cache pages you wanted to use when you get back to the machine. It doesn't have to though.
This might be a bit autistic of me, but I don’t trust that random processes sitting on 100% cpu are serving me in any way. I don’t think I want this sort of background process to run on my computer at all.
Are those programs written well, or are they using so many cycles because they’re inefficient and slow? And when did I ever opt in to this? Spotlight has slowly gotten more and more horrible over time. Half the time I use it to invoke system preferences it can’t find it. Or it can’t find the applications folder. If Spotlight is this terrible, why is the hard disk indexer so busy? Is it any better engineered than spotlight? I doubt it. Likewise, I don’t want photoanalysisd looking at my photos. I don’t use that “photos by person” feature. Why does it use hour upon hour of cpu time to make this feature available - just in case I use it later I guess? Get lost.
I really wish Apple stopped adding random crappy features to macOS that I don’t use - but which burn cpu cycles. Instead, fix your shit. Indexing is fine if you make spotlight actually be good again. Photo analysis is useful if I decide it’s useful and turn it on. And maybe if Xcode and SwiftUI weren’t such a buggy, crash ridden, undocumented mess, then maybe, maybe, I’d trust you more to run random background processes.
As it stands, I don’t trust Apple - particularly their application teams - to be good custodians of my cpu.
The “bug” here is system activity I’m not deliberately invoking.