Significant trouble with using R9000 as an additional access point

[TTait]

High all,

So following an installment of a new gateway by xfinity I was very unhappy to realize how limited the customization options were with the unity, the model of our current gateway is CGM4140COM, and the coverage of our home was fairly spotty as well so I figured it would be reasonable to continue using our old R9000 as the primary AP connecting it to the gateway and using it instead. The problem I've run into however is that it seems the R9000 speed is severely throttle and I cannot for the life of me figure out what it is. When running a hard wired connection to the xFi gateway itself I get speeds upwards of 475mbps, but when running a hard wired connection to the R9000 which is also connected to the gateway that drops to around 70mbps. I have always liked the customization options available on the netgear router and the coverage is a lot better than the gateway because of how easy it is to relocate, but I just cannot seem to find a solution to this strange throttling problem. Any ideas would be very appreciated.

Edit: To briefly clarify, the firmware is fully up to date and QoS is enabled with the hard wired device being set as the highest priority.

[plemans]

A couple questions. 

1. is the xfinity gateway still in router mode? OR has it been put in bridge/passthrough mode?

2. is the R9000 in access point mode? or router mode? 

3. Have you tried replacing the cables? 

4. If you're speeds are over 250mbps, disable QoS, access control, traffic monitoring, and parental controls. Then reboot the router.