How did you measure this? Iperf3? Info in LuCI or Ubiquite WifiMan?
Check out Best practices and tools for measuring wifi performance for a rough introduction about how to measure wifi performance. (I still have to refine this thread a little)
Things you could try:
- Have you tried to disable Software flow offloading?
- Have you tried lowering transmit power? In certain (but not all) circumstances it actually can increase transferrates.