Problem about preview and finder

Please download and run Etrecheck  It is a diagnostic tool that's very useful to us in finding problems. Also it will give us further specs on your Mac. After it runs post the log file here. It will contain no personal information. Allowing full drive access will improve the quality of the report.

To post the log file click on share report and use the Page icon in your reply window.

Yes, it looks a lot better! You can see how much faster you're running by the runtime differences in your Etrecheck report. 5:28 vs 3:06.

The only two files I still see are these:

/Library/LaunchDaemons/com.mackeeper.MacKeeper.plugin.AntiTheft.daemon.plist

/Library/Application Support/MacKeeper/MacKeeperATd

So just go to your /Library/LaunchDaemons folder and delete com.mackeeper.MacKeeper.plugin.AntiTheft.daemon.plist

Then go to your /Library/Application Support folder and delete MacKeeper/MacKeeperATd.

(You may be asked to authenticate here because they are system files)

Also, you can clean up the files under "Clean Up" from within Etrecheck by using the CleanUp button.

Well, neither Avast or Kaspersky uninstalled completely. Especially Avast which is all over your Mac. You'll need to uninstall them as per the developers instructions. You may even need to re-download and install them to uninstall them.

https://support.avast.comhttps://www.apple.com/macos/security/

Not sure which Kaspersky you have but you can search for "Kaspersky uninstall Mac"

EDIT: You also have MacKeeper which as bad as malware. Uninstall that also.

No Anti-Virus software or so-called “cleaning” apps are needed or recommended for Mac OS. They can conflict with Mac's own built-in security. At best they will slow your Mac by using unnecessary resources and at worst will bork your entire system. Your Mac has all the built-in protection you need.

https://www.apple.com/macos/security/

It would be my best guess that this is the source of your issues.

After you uninstalled as much as possible. Post another Etrecheck report and we can go after anything left over and delete manually.

If it works as expected in Safe Mode, then the first place to look is in Login Items. Go to System Preferences > Users & Groups > Login Items and use the minus (-) button to delete them. (You can always add them back with the (+) button.) Then test.

Startup in Safe Mode and see if it works as expected.

Use safe mode to isolate issues with your Mac - Apple Support

You're welcome, looks great. Just keep an eye on your battery as it looks like it will need to be replaced soon.