Sometimes the performance issue can be related to Metahumans and their hair, can you give this a try to see if it improves performance for you? Additionally, the megascan assets use a large amount of video memory at their highest quality settings (with the highest usually meant for cinema), using lower-quality versions of the assets can also help improve performance at this stage.
Since you appear to be having issues with any scalability settings, the issue could be beyond UE5 itself and instead be with how your PC is setup (potentially thermal-throttling or mis-matching of parts). UE5 is a demanding engine to run even at base-level, and while the specs of your PC far exceed the system requirements, it is understandable that a poorly configured setup could be the source of the issue. That said, a first step would be checking if your video card drivers are up to date.