Certain websites SUPER SLOW on any Xfinity router??

I have a number of personal custom websites [hosted by FatCow] and when I access them by any internet provider they are super fast and responsive and everything is perfect. The second I switch to either my personal Xfinity Wi-Fi router at home *OR* any public “xfinitywifi” router all of these websites are ridiculously slow and unresponsive and it takes like 10 seconds to load anything at all. Then when I switch back to cellular or any other Wi-Fi, everything is perfect again.

I have reset all browser caches including cookies and DNS and literally anything else…I have also tried this with three different machines with three different operating systems, and both Chrome and Safari on all three of these, and the exact same thing happens each time.

Is this a known issue?  Is Xfinity somehow super throttling traffic to these FatCow websites for some reason?  Is there any way to fix this??