Go step by step and test.
1. Startup in Safe Mode and restart normally using the “Restart” button at the bottom.
https://support.apple.com/kb/PH25617?locale=en_US&viewlocale=en_US
2. 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.
3. Remove Cookies and test.
Safari > Preferences > Privacy > Cookies and other website data:
Click the “Manage Website Data” button.
Remove all cookies except ones from Apple, your internet service provider and banks.
4. Disable Extensions and test.
Safari > Preferences > Extensions
Select and disable all extensions and test.
Enable Extensions one by one and test.
To uninstall any extension, select it and click the “Uninstall” button.