Just to be clear, link speed is not a speed test. Your link speed is also dependent on the STAtion\client you are connecting to, as well as the channel width for your MT300A. You may also have different link speeds for each radio or connection. There is also an up and a down link speed, for a total of 4 values.
Use the Advanced Config (Luci) and go to Network=>Wireless. Choose Edit for one of the wireless connections. (One will say linked to …, use the other). Set the channel width to 40 from 20, if it’s not. The devices at the other end should have similar configurations to get the best “link speeds”, or the MT300A will run at the lower of the 2 speeds (at best). Link speed will probably fluctuate.
Did you try the USB key on an extension?
For your speed test use either
http://www.speedtest.net/ (IF you have a choice of test locations try to be consistent)
Internet Speed Test – AT&T Official Site (personal preference)