Safari is extremely slow afer upgrade to Mojave

Go step by step and test.

   1. General

       Restart your Router.

       Startup in Safe Mode. https://support.apple.com/en-us/HT201262

       Launch Safari  and test.

       Restart normally by clicking the “Restart” button at the bottom.

   2. Disable all extensions and test.

       Section: Manage extensions:

       https://support.apple.com/guide/safari/use-safari-extensions-sfri32508/mac

3. Safari > Preference > Advanced

      Checkmark the box beside "Show Develop menu in menu bar".

      Develop menu will appear in the Safari menu bar.

      Click “Develop”  to show the dropdown.

      Click "Empty Caches" from the dropdown and test.

      https://support.apple.com/guide/safari/use-the-safari-develop-menu-sfri20948/ma

4. Check for adware.

      Run  the latest release of Malwarebytes for Mac to remove malware/adware, if installed on your Mac.

      For instructions:  Install Malwarebytes for Mac          Uninstall Malwarebytes for Mac

      Click the “Scan Now” button. Once done, quit Malwarebytes for Mac.

      Restart the computer and relaunch Safari holding the shift key down.

Try setting up another admin user account 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        Pastebin

Etrecheck – System Information