My mid year 2012 macbook pro is running Mojave 10.14.6 and after running the 9-25-20 secrity update it takes forever to start, shutdown or load programs. It will freeze and then unfreeze. It runs the same or worse when started in safe mode.

What OS were you on before???

Do you have a Time Machine backup to Restore to/from?

https://support.apple.com/guide/mac-help/revert-to-a-previous-macos-version-mh15216/mac

If not…

https://www.imore.com/how-downgrade-macos

Many people having those problems.

Try the standalone in Safe Mode, but not using Firefox...

https://itunes.apple.com/us/app/macos-mojave/id1398502828?mt=12

Safe Boot, (holding Shift key down at startup).

Keep in mind Safari 14 eliminates Top Sites. Also, the issues you are having have been experienced by many.

Both Safari 14.0 and Security Update 2020-005 were pulled and then replaced as of this evening to address many of the issues people are having. great resource

See this:

https://mrmacintosh.com/mojave-2020-005-security-update-causing-major-problems-updated/?utm_source=rss&utm_medium=rss&utm_campaign=mojave-2020-005-security-update-causing-major-problems-updated

I think it’s because the new system update, I am having similar issues

Yes, it is the Security/Update on several Macs, did you try reinstalling Mojave while in Safe Mode?

Same issue for me on late 2013 iMac - completely unusable in safe mode or regular boot. Had to run recovery mode to reinstall. this occurred twice and I had to turn off auto-update

You're welcome, take care my friend. :)