If Safari functions as expected in another User Account, then the problem is specific to your regular user account. That would narrow it down to any Safari plug-in or extension unique to the regular User Account.
I suggest focusing on extensions first. If you have one or more extensions installed follow these steps.
Open Safari Preferences>Extensions and disable each extension by clicking on the extension, then unchecking the "enable" box. Then, restart Safari.
If performance improves, then return to Safari Preferences and enable one or two extensions. Restart Safari. If CPU activity is normal, then continue down the list following the above sequence until you find the culprit.
As with any major OS or application update, it's up to the 3rd party developer to bring their product into compliance with new system requirments.