I have had this for all sites I have done in webflow, but its not related to just webflow. You can take pretty much any site and see this sort of rating for mobile, heck I even tested Microsoft and they came under a satisfactory score. If you are hitting 100/100 what more can you do for mobile?? I think its just really really brutal scoring, that is to protect places with very limited net speeds and low quality devices.
We had the idea that this gap on mobile can be fulfilled with AMP pages. Yes, they don’t range as the main speed factor now, but maybe Google still save some points for AMP enablement. But it’s hard to check it on Webflow, as it doesn’t support it