MacBook Pro very slow

1 - Restart in Safe Mode. This will perform a Disk Repair, clear cache files and only load Apple Software, extensions and fonts. The boot up will be slow and can take some time - Normal.

2 - Does the issue present in this mode ?

3 - Sometimes a Safe Boot followed by a Normal Boot will just put things right.

4 - If not - there could be something in the main User Account playing up. To further isolate this - Set up users, guests, and groups on Mac. Then log out of the Main User account and log into the dummy account and test again if the issue persists.

5 - If the issue is present in the dummy account - then, this appears to be a System Wide issue on the computer.

Understand, we are all working Remotely from all around the world. 

Therefore, we do not have the On-Hands experience the User ( you ) has with this computer.

The next best thing for us to having a semi - actual On-Hand experience on this computer is to follow the steps below

Download the Application Etrecheck directly from the Developer.

The application is free or paid from added features. 

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.

As newer macOS versions are released they get more demanding, which older computers obviously will have a hard time handling. One idea is to downgrade the computer back to an OS that the computer handles, or purchase a new laptop.