Sure, but it's going through your router, right? So if you were torrenting and someone else jumped on the xfinity wifi and started torrenting, it would affect your ability to use your bandwidth, right?

As another post said, only jitter (and only minorly).

Bandwidth would be unaffected. Or more exactly, on WiFi (but not Ethernet): as affected as if your close neighbor used that amount (since WiFi itself has limited bandwidth). The connection from the router to the cable modem has lots of headroom, so that would be unaffected, as would wired connections.

yea, xfinitywifi affected our jitter test. We called Comcast to disable it & our jitter went down.