I have the same issues. The latency is completely unacceptable, usually over 100 milliseconds. No Wifi calling so I have to use Google voice if and when I can find an Xfinity hotspot. I've found several areas where AT&T used to work, if it really is using Verizon towers, I'm not getting priority access. To make matters worse Xfinity's coverage map COMBINES Wifi hotspots with LTE giving the consumer the illusion of great coverage. Seriously thinking about leaving Xfinity. I've contacted them and never get a return call. They know the network is isn't as good as Verizon, and they have zero knowledge!!!!
XM uses Verizon towers. There is no if. I've noticed latency is higher on XM v Verizon which to me would mean Verizon is prioritized which makes sense . I have both carriers and speed on Ookla are identical with latency being the only difference.