Apple has recently made the supplemental update 10.14.6 update available to macOS Mojave users.
One notable change is that Safari 14 is back after being removed because it was causing issues for some. The 2020-005 security update in September seemed to add problems, including various error messages and slowdowns.
The Safari 14 version is reported to stop working for some as well. With the supplemental 10.14.6 fix the issues related to Safari 14 and the Mojave 2020-005 update are addressed.
If you’re one of those who are affected by Safari 14 or the 2020-005 Security Update, then it’s best to get the supplemental update as soon as possible. Head to the macOS System Preferences app, then access the Software Update menu and click the ‘Install Now’ button. The download file is about 70 MB and should only take a few minutes.
Afterwards, reboot and test out Safari 14 and see if there are still slowdowns or performance issues.