I was having same issue with Nighthawk. Have a 110Mbps connection (from Comcast and also through an SB6141) but was only getting 10 Mbps (or less) passthrough on wireless 5G band (at start of test speed would sometimes be as high as 50 Mbps, but over a few seconds would drop to nothing..)
Wired speeds were fine. Tried several different channels and factory resets of the router, wireless speed was still slow. Tried changing channels (I'm the only router on the channel). Logged onto Netgear website and submitted a support ticket.... within 2 hours they emailed me back with an RMA and the next day a new unit was being FedEx'd to me. Tech support rep said it's a known hardware issue in some units... I was super impressed with the customer service, it seriously was a no hassle replacement that took all of 15 minutes of my time.
UPDATE: I received the replacement router within a few days. Awesome. Except it had the SAME PROBLEM. As soon as the new router was on and I logged in, I upgraded to the latest version of firmware, did a factory reset, and ran Speedtest. And I had the same problem: internet passthrough speed on all my wireless devices was awful. Less than 10Mbps when I have a 110+Mbps connection. (Wired speeds were fine, so was not a provider or modem issue).
Contacted support again and they instructed me to manually download latest firmware on laptop and manually upgrade router, doing a factory reset both before and after the upgrade. They also told me to try turning off WMM (made no difference in passthrough speed, but did lower my laptops link speed from 433Mbps to 54Mbps!).
The only thing that changed since I first got a Nighthawk back in July 2014 (and I ran speedtests then and had a 110+Mbps wireless speed at that time), is that the firmware has upgraded several times... which got me thinking...
I downloaded from Netgear support the last 5 versions of firmware for the Nighthawk.
http://downloadcenter.netgear.com/en/product/R7000#searchResults
I then manually downgraded the Nighthawk to each previous version (doing a factory reset both before and after each installation) and ran Speedtest each time. Once I got down to Version R7000-V1.0.3.24_1.1.20 (Released 2/12/14) and ran Speedtest, the PROBLEM WAS SOLVED. Wireless speeds on all devices (laptop, HTC One M7, and iPad Air 2) were all back up to 110Mbps or more!!!
Hope this experiment helps out someone else... if your're experiencing slow internet passthrough speeds and it's not your link speed (i.e. radio interferrence) that's the issue, there seems to be a serious problem with the last couple newer versions.. Try downgrading to firmware version R7000-V1.0.3.24_1.1.20