Hello, I have 3 R510's in my home. 1 for each floor, ceiling mounted and placed central to each floor. None of the clients on our network can ever achieve more than 50Mbps. My PC which is 15ft around a slight corner from the AP got 30 Mbps down and 80Mbps up on an AP to Client speed test.
FYI, that post was referring to R610 and you have R510, big difference. What kind of speed is your ISP guaranteeing? Are you getting Fiber Optics to the home? Do you have the latest firmware for R510s? I had similar issue with Comcast and i debugged it for 6 months. Ended up being multiple issues. Comcast was not delivering 1Gbps, but kind of talked about it.... Switch over to AT&T Fiber Optics at 1Gbps and also upgraded my APs(one upstairs and one downstairs), keeping my existing switch(new modem/router from AT&T), and upgraded to R610s. Now my wifi speed 490Mbps Down/500Mbps Up typically around the house. Goes up to 550-600Mbps down at times.
I had Charter come out and do a speed test and I saw them get 1Gbps at the modem. I am getting Windstream fiber 1Gbps service soon moving the cable modem down to 200Mbps and will eventually have some simple load balancing.
The speed test results I supplied are from AP to client, should not be dependant on WAN network interface speed.
From my earlier experience, the R510s do not have great antennas or you may have lots of interferences. How is your RSSI/SNR? May want to play with it and find a less noisy(less intereference) channels. Careful what these cable guys tell you, they might tell you one thing and showed you another. My experience with Comcast...