...Activity Monitor is far from easy to deal with as so many culprits are coded not to take blame. .. that 's by guess on that so far..
First, if your Internet is slow, you're probably more likely to find the problem living all around you than in Activity Monitor, i.e. Internet slowdowns are considerably more likely to be ISP, e.g. heavy traffic, related than the result of something going on on your iMac.
I've got zero idea what you mean by "coded not to take blame;" AM shows what's going on impartially. If a process is using CPU cycles, AM shows it, and if it's using so few cycles that it doesn't rise to the top of the list, it's probably not slowing you down.
The one thing that MAY be slowing things down is your iMac itself, and to judge the possibility we need some info beyond the idea we've already got of which apps you're running:
- Exactly which model iMac are you running?
- How big is your internal drive?
- How much empty space have you got on your internal drive?
- How much RAM have you got?
The answers won't necessarily point to a problem, but they may, by giving us a perspective on what 90% CPU means on your particular machine.
We very often have trouble addressing your questions because you very often don't ask complete questions, i.e. you omit important info such as that I just asked for.
PS:
This (*) is an example of what you see in AM when something on your Mac is responsible for your slowdown.
The 635% is 53% of the possible 1,200% available from my 6 hyper-threaded cores, but I'm not sure if that agrees with what joemike said in his last post.
(*) OK, I confess. That 635% is a situation I contrived by taking advantage of a bug I"ve uncovered in macOS 10.15.5 (at the least). I haven't filed a bug report yet, because although it's reproducible, I haven't been able to figure out how to reproduce it reliably.
