I used an wifi analyzer to see whether there are other wifi devices in neighborhood using the same channel as R700 in 5GHz band is using. There are many routers in 2.4GHz band but in 5GHz band only my R700 is active. I unplugged all electric devices in my home and had only the ATT-UVERSE modem, R700 and ipad air 2 powered up. I tried many locations as close as 5 feet from router. I tried also many orientation of the the ipad. All results are very similar. At the beginning of the test I achieve around 10 MB/s than I achieve close to 40 Mb/s for 10 to 15 seconds, than the data throughput breaks down to around 10 Mb/s for couple seconds and than it goes back up.
This explains, why I can't watch youtube movies at high resolution.
There is no RF-interference in my single floor, open home. It is just R700 unable to to maintain the throughput.