Speed drops when NETGEAR r8000 connects to Xfinity

Hi experts

I recently upgraded to 1G with Xfinity and I installed XFinity Modem and used the built in wireless router . Speed is fabulous and close to 1G. 

however when I connect the NETGEAR r8000 to the comcast modem and use the wireless function of NETGEAR, the speed drops by half. I have tried disabling QOS and enable and no change in speed and remains at 500Mbps.

when I run speed test from the app within the NETGEAR it shows 922Mbps but when I run it over the app on my phone, it still remains at 500Mbps. 

what could the problem be?