If you want to check out performance issues on the AP, associate to the ap with a laptop and if possible, run the telnet emulation on your laptop also. Other than that, you have performe your basic troubleshooting.
Slowness could be an indication of poor RF signal quality or interference. Have you had the area in question surveyed? If not, look for multipath, noise, and cochannel interference. I am betting if you use Omnipeak Wildpackets that you will see multiple retries. If this is in fact the case, you need to find the reasons why.