Did you apply the 2020-005 SecUpd recently, that has messed with many Mac's speed, & the cure is to reinstall Mojave, hopefully in Safe Mode.
See if you have unknown Profiles.
To remove a configuration profile in macOS:
- From the Apple menu, select System Preferences....
- From the View menu in System Preferences, select Profiles.Note:
- Profiles won't be visible until you have at least one profile installed.
- Select the profile you want to remove, and then press the - (minus) button. Click Remove to remove the profile.
Safe Boot, (holding Shift key down at startup), does the problem occur in Safe Mode?
Safe mode attempts to repair Disks & clears lots of caches & loads safe Drivers, & prevents loading of 3rd party extensions, so if Safe Mode works try again in regular boot.