Please remember your Mac is a vintage machine and is likely running apps that are vintage as well. I would start by ensuring all of your third party apps are up-to-date. That means you will need to launch each one and check for updates. If you don't see and update from within the app, navigate to the app's website and research it. If you have apps that you no longer use (I'd be very willing to be that is correct) then uninstall them per the developers instructions.
If you still have a problem, then please do the following:
Please navigate to www.Etrecheck.com and download the free version of EtreCheck. Once you have you downloaded the app and installed it, please run the report and save it. This report will help us get a good idea what has been installed on your system and help us be able to diagnose what may be wrong.
When you have your report, you can attach it when you reply to this message and we can then review it and help you determine what is needed to get your system running well again.
For instructions on how to download your EtreCheck report and attach it to your reply to this message please click https://discussions.apple.com/docs/DOC-250000211