Hello,
to analyze performance problems, you have to deliver way more information.
What means "slow"? Slow response times, low throughput?
When have you installed the AP3805i? Is this a new installation? Does the problem exist since the beginning or have you performed any changes or updates in the meanwhile?
Are all users affected? Can you test the throughput by yourself?
Is this an open network or does it use PSK or 802.1x authentication?
Is the traffic bridged at the access point or at the wireless controller? If it's bridged at the controller, does the controller reside at the same location/ in the same local network as the access point?
One option that could limit the speed to 54Mbit/s is, if you have not activated "WMM". You can select that option under VNS -> WLAN Service -> QoS
You could check how the client connects under Reports -> Active Clients (check speed, RSSI and so on)