activity monitor
how interpret activity monitor to determine why iMac 2017 is very slow?
iMac
One method I use is to select the CPU tab and sort by %CPU in descending order.
Activity Monitor User Guide for Mac - Apple Support
Another fundamental troubleshooting technique is to use "Safe Mode": How to use safe mode on your Mac - Apple Support. A Mac is not pleasant to use in that mode for very long, but it should not be objectionably slow.