hemlockiv How would I diagnose that?
You can see overall CPU usage with an app like so
You can't have an itemized list; however, you can perform your experiment on the top of an even hour (E.g 2, 6, 18,) as the measurement periods under battery usage are partitioned at that interval.
Or you can get a hint by seeing what user installed apps are currently running: https://imgur.com/FmgMd7q
You can stop some or all of these to see if there's a change in speed.
Actually, this doesn't list every background app for some reason. Temporarily enable developer options and go here:
developer options > running services
After a GOS update, CPU usage will be high while it optimizes apps--that's normal--but afterwards should still be sluggish as you mentioned.