Hi @Keith_Li,
We will need to do a bit of digging to determine why the problem is appearing on that one AP. You can start by ruling out a few common factors that often cause wireless latency. Distance from client to AP, channel utilization and SNR are all things that would be worth looking into.
I would advise starting by selecting that AP and reviewing the connection logs. What SNR and Time-To-Connect values are you seeing? Is only one client having issues, are or all clients roaming to this AP having issues?
How long ago was the site survey taken?