Ok.. let me suggest a few ways to measure things.
Check the lights on the Xfinity if it shows ethernet speed..
The airport utility on iOS device will also give you that speed.. I don't think you can get it out of the Mac version at all. (with some nasty hacking)
Just load the iOS version onto your iphone or iPad and tap the icon of the TC..look at the connection and it should tell you the link speed.
Another way would be to copy a large file to the TC hard disk.. in its current location.. via wireless.
Then turn off the wireless in the TC and copy the same file to the TC again via the Xfinity router. (that is passed via the ethernet link and you will quickly see the difference if any).
If it is clear the ethernet is slow.. then plug the TC into the Xfinity router with a short patch cord.. and turn off its wireless.
Do the test again.. if it now speeds up the cable is bad.
There are perhaps indications of speed of ethernet in the Xfinity router.