Hi, I am having a similar issue here. I have a N66U and a N12HP, both having the same speed problem. the capacity of the wifi/lan signal is definitely fine as I tested with bit torrent I could get close to full speed. However, for http downloads, this issue is pretty serious. I can't even stream youtube.
Here is how I troubleshot:
I tested with speedtest.net, google drive file download, also my ISP's own speedtest.
1st case: connect pc directly to modem and use PPPoE dialer. full speed
2nd case: connect N66U/N12HP to modem and then Lan/Wifi to PC, all default setting. full bt speed, 1/5 http speed. so the signal capacity is fine, but something wrong with the PPPoE protocol or something.
3rd case: connect N66U/N12HP to modem and then Lan/Wifi to PC, set IP pool starting address to 192.168.1.100 (instead of 192.168.1.1 by default) http speed improvement, but still not full speed.
4th case: connect an old router (linksys wrt160v2) to the modem then set the routers to AP mode. Full speed.
I can conclude there must be something wrong with the PPPoE settings/code.
Please help! I really regret buying this N66U yesterday, I should have gone for a linksys!