do you see this speed issue on other end-devices/clients too?
How did you test the throughput? With iperf?
It can be a configuration issue. If you look into the bsap firmware release notes, there are several points impacting the performance you should pay attention on.
For example, if you use AES and TKIP encryption together (mixed mode), clients will use TKIP by default, which limits the transmit rate to 54 Mbps.
A good troubleshooting step will be to verify if the throughput problems only occur on 2,4 Ghz or 5 Ghz Radio.