internet speed

I've never been able to get the speed I'm paying for. I'm paying for up to 1100mps but am rarely able to get even 500. I'm hardwired to my router with a CAT 6 cable and my connection from my modem to the router is also a CAT 6 cable. Both my modem and router are state of the art and rated for up to 2000mps. There are no splitters on the line. When I contact xfinity they tell me they don't see the problem. I'm wondering if the problem is with the building I'm in. My service is associated with my coop, but I pay for the 1100mps service separately. Could that be why I'm not getting the speed promised?