Hi
There could be a number of reasons why you are seeing some latency on your SSID.
One issue is the link speed. Try updating to a 1GB link on the AP.
You did not mention the AP loading. Howm many users are typically on a single AP.
Are your WLAN configured to bridge LOCAL or TUNNEL.
Finally, you need to look at the data coming out of the AP. You can do a PACKET CAPTURE on the AP interface or mirror the port on the switch and review the WIRESHARK capture.
Finally, make sure you are on the latest available firmware for your equipment