safari slow after upgrade to Cataline

Safari/Preferences/Advanced - enable the Develop menu, then go there and Empty Caches. Quit/reopen Safari and test. Then try Safari/History/Show History and delete all history items.  Quit/reopen Safari and test. You can also try try Safari/Clear History…. The down side is it clears all cookies. Doing this may cause some sites to no longer recognize your computer as one that has visited the web site. 

Go to Safari Preferences/Extensions and turn all extensions off. Quit/reopen Safari and test. If okay, turn the extensions on one by one until you figure out what extension is causing the problem.

Are you running any anti - virus programs? Using a VPN?

Try setting up another admin user account in System Preferences/Users & Groups to see if the same problem continues. Please post back on whether or not this worked. Also try the Safe Mode. Please post back on whether or not this worked.

Isolating an issue by using another user account 

Safe Mode - About

If it works in the Safe Mode, try running this program when booted normally and then copy and paste the output in a reply. The program was created by Etresoft, a frequent contributor.  Please use copy and paste as screen shots can be hard to read. Click “Share Report” button in the toolbar, select “Copy to Clipboard” and then paste into a reply. This will show what is running on your computer. No personal information is shown. If the log won’t post, try posting it in Pastebin and provide a link in a reply. After pasting the report in a PasteBin page, go to the top of the page, and copy the address in the URL bar. Paste that in a new reply.        Pastebin

Etrecheck – System Information

I've had the same issue since I upgraded to Catalina. It seems this was linked with my anti-virus program (Browser worked perfectly on a basic guest session)

After updating the AV to its latest version, everything went back to normal.

Anti-virus programs aren't needed on Mac computers. They often cause problems. You might consider uninstalling the program using the Develop's uninstall instructions.

It can't hurt to try Malwarebytes.