My internet is very fast if I plug my laptop into the router with Ethernet (40 mbps). If I put my laptop right next to the ASUS RT-N66U router, it's ok (18 mbps). If I move it 10 feet away (no walls), it drops to next to nothing.
I live in an apartment.
I have tried everything I can think of, rebooting, trying 2.4 ghz, 5 ghz etc.
I had awful reception when I was using comcast's router as well so I'm no 100% sure it's my router but who knows.
I read both of these:
http://forums.smallnetbuilder.com/showthread.php?t=14014&page=2