It happened to me again today this time on the 5G band with my iPad Air.
Internet becomes extremely slow to the point it takes OVER two minutes to load. Turning off wifi on the device and turning back on seems to help momentarily before it slows down again. I should note all devices have assigned IP's. Doing a soft or hard reset of the device does not fix this. Switching bands or rebooting router corrects this issue.
Issue is similar to this users post:
http://forums.smallnetbuilder.com/showpost.php?p=138894&postcount=75
I could be way off base here but think on a different router RMerlin mentioned there's some drivers for apple devices that may need to be updated, not sure if I recall his post correctly or if it even would apply to this router.
As I stated above Im a registered Apple developer and have access to higher end support and each time I've reached out to them they've concluded the same as me as this is a router issue and my own experience as noted here points to the same conclusion.
I'm not a router ninja so don't know how to pull logs if any would even be helpful but hoping my description helps.
As stated previously my router is configured with default settings except I have assigned IP's and I have adaptive QOS turned on and my iPad Air has the highest priority. All other settings are defaulted.
From what I know about the router and from what I'm seeing the 5ghz band driver needs work because if simply switching from the 5 to the 2.4 and that fixes my issue when im 30 ft away from router with nothing but Sheetrock and wood between the router and I then to me it's the driver.
Hope this helps in solving this issue. I seem to run into it quite several times a day if I try to stay on the 5ghz band.