Go step by step and test
1.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. Launch Safari holding the shift key down and test.
3. Turn off extensions and test.
https://support.apple.com/guide/safari/use-safari-extensions-sfri32508/mac
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.
5. If the steps posted above don’t help:
Download EtreCheck: https://etrecheck.com/maspro and post the report here.
Click, “Free Download” button and then “Click to download” button.
Open Downloads folder, click on it to open, and then select ”Open”.
Click on the bouncing EtreCheck icon in the Dock.
“Choose a problem” from the popup menu box, and then “Start EtreCheck” in the dialog.
Click “Share Report” button in the toolbar, select “Copy report” .
Paste the report when you reply.