Go step by step and test.
1. Empty Caches
Safari > Preference > Advanced
Checkmark the box beside "Show Develop menu in menu bar".
Develop menu will appear in the Safari menu bar.
Click Develop and select "Empty Caches" from the dropdown.
2. Delete Cookies
Safari > Preferences > Privacy > Cookies and other website data:
Click the “Details” button.
Remove all cookies except ones from Apple, your internet service provider and banks.
3. Disable Extensions and test.
Safari > Preferences > Extensions
Uncheck boxes beside “Enable Extension” and test.
Enable Extensions one by one and test.
To uninstall any extension, select it and click the “Uninstall” button.