I have a Comcast router XB-6T and it's suppose to be better/faster than using my Time Capsule/802.11ac as a router.
Better/faster is very subjective. You need to define what "better" and "faster" mean to you. But for comparison:
Both are 802.11ac devices
Both have Gigabit WAN connection (the XB-6 claims "a maximum" of 1 GB ...