How are you judging the speed? By the device status GUI or by an actual file transfer benchmark?
Also check out item number 5 in smallnetbuilder article "5 Ways To Fix Slow 802.11n Speed".
Fix #5: Don't use Channel Bonding
If you are changing the Channel Width or mode from the default 20 MHz to the 40 MHz (or "Auto 20/40" mode in some routers) channel-bonding mode, you could be reducing, not increasing your throughput.
The Channel-bonding trick can provide a 10 to 20 Mbps throughput increase, but usually works best under strong signal conditions. As signal levels drop, using channel bonding becomes much less effective in providing a throughput boost.
Check out location D in the Wireless Chart plot below, which shows that using the 40 MHz bandwidth mode reduced throughput by around 75%! (Channel bonding in Location B didn't work so well either.) I have also found that with some routers, switching back to 20 MHz bandwidth yields a connection when I couldn't even get one using 40 MHz mode.