Have you tried running the free version of Malwarebytes? It was developed by a long time contributor to these forums and a highly respected member of the computer security community.
If that's doesn't work try this: download and run Etrecheck. Copy and paste the results into your reply as described below. 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.
IMPORTANT:
Before running Etrecheck assign Full Disk Access to Etrecheck in the first window that comes up so that it can get additional information from the Console and log files for the report:
Also click the About… vurron and read the info to further permit full disk access.
Copy the report
and use the Additional Text button to include the report in your reply.
Then we can examine the report and see if it identified the culprit and tells where it is.