Does the same issue present when booting in Safe Mode. On restart hold the Shift key until the Apple Logo is presented - then release.
Safe Mode with do a Repair Disk, clear cache files and eliminate all third party software, extensions and fonts from loading. The boot up will be slow - normal.
Once logged in - run this was for some time to test is your issue is presented.
If the slowness and can not open any application is Not Present in Safe Mode - this may indicated some piece of third party software is the root cause of the slowness in normal boot.
Then would suggest the below course of action >>
Suggest downloading the Application Etrecheck https://etrecheck.com directly from a well Respected ASC Contributor.
The application is free or paid from added features.
Run the application with Full Disc Access ( Security & Privacy - Full Disc Access ).
It will take a Snap Shot - both the hardware and software.
The Report will Not Reveal Any Personal Information.
Post back the Full Report - copy and paste - using the Additional Text Icon ( 3rd Icon to last )
We can have a look at the report for possible issues and may have possible suggestions to resolve the issues.
Any Third Party Applications that will interfere with the normal operation of the OS, alter, modify, remove or delete or attempt to do so is an invitation for disaster and may require a Reinstallation of the OS.
This includes AntiVirus, Cleaners, Optimizes, UnInstaller etc.
