It isn't uncommon to see that the theoretical max speed of wireless does not match the real world speed.
However there are other options that you can play with in the advanced settings in DD-WRT that might improve your speed.
Other options that might be of value include enabling Frameburst and afterburner. Make sure you test as this could potentially make the performance worse. Also read up the max values suggested for your transmit power. For my router an asus rt-n16 its different, its in dB not mW which is why it is a much lower value. This is an exception. Your router, which I've had before, is definitely in mW. Generally the default value for this is just fine.
You may also want to change your wireless adapter settings on your computer. This is outlined here and also here.
If you have the options there for frameburst or afterburner enable it. You'll want to test after each of those changes.
If you have a transmit power option set it to the highest or max performance rather than max power savings. There shouldn't be any negative effects from this except for battery life.
For future reference. I left this for others but I saw you did this already in your edit:
As mentioned in the other comment. Wireless channel is important because you don't want to be on a channel that interferes with a neighbors wifi network if possible. The best channels to select are 1, 6, 11. I prefer 1 and 11 as the frequency of channel 6 is close to the operating frequency of microwave ovens - 2450Hz -and I've found interference with it personally. To see what channels other wifi routers in your area my be running at go to Status then click wireless.
Then at the bottom click "site survey"
Here is a list of my neighboring wifi routers. The closer the noise level is to -100 the worse the signal is, meaning they're far away and likely not to cause interference should you select that channel. You can click on the titles to sort by that column value. It won't show your router.
For reference this is what mine looks like. I would say I have moderate interference.