Please download and run Etrecheck. The free version is sufficient.
Copy and paste the results into your reply. Etrecheck is a diagnostic tool that was developed by one of the most respected users here in the ASC and recommended by Apple Support to provide a snapshot of the system and help identify the more obvious culprits that can adversely affect a Mac's performance.
Copy the report

and after clicking on the Reply button use the Additional Text button to paste the report in your reply.

Then we can examine the report and see if we can determine the cause of the problem.
