Hi rimba56!
Good question, this article covers a lot of what to look for:
If your Mac runs slowly:
https://support.apple.com/guide/mac-help/if-your-mac-runs-slowly-mchlp1731/mac
There’s a lot of good input on that article, including checking storage, repairing the disk in Disk Utility, and then some.
Another article on the bottom of this one:
View disk activity using Activity Monitor:
https://support.apple.com/guide/activity-monitor/view-disk-activity-actmntr1005/mac
This May give clues as to the root of the problem. Maybe it’s just a runaway process that needs to be quit, an app causing issues because it’s incompatible and needs to be updated, uninstalled, or just prevented from launching automatically, etc. or something else, this is a good way to check.
Hope those help!