That data rate and the data transferred does not explain your issues.
Do you have a floor plan that shows where your access points are mounted?
- You have access points mounted 3 meters on the wall. They should be mounted on a ceiling, or if they are mounted on the wall, they need to be pointing down. If they are not pointing down, your clients on the ground, especially right under the access point will have a problem connecting, due to the pattern.
- You have the transmit power on your access points at 21, which is too high, since you have them mounted at every 20 meters. You should change the ARM maximum Transmit to 18 or 15 and the ARM minimum Transmit to 12: http://www.arubanetworks.com/techdocs/Instant_41_WebHelp/InstantWebHelp.htm#UG_files/ARM/Configuring_ARM-Features.htm
If you have the transmit power too high, that will cause interference with other access points and that will cause or contribute to your performance issue. If you only have a few users, they might get good performance, but as more users connect and they add to the interference, your throughtput will go down. Having access points at the highest power will also cause roaming issues. Also you have an access point that has high utilization (78) that you might have to look at see if there is other interference like a Microwave or other wireless devices that could be causing your issue.
If you can, make sure that ARM Client Match is turned OFF (http://www.arubanetworks.com/techdocs/Instant_41_WebHelp/InstantWebHelp.htm#UG_files/ARM/Configuring_ARM-Features.htm) and you are running Instant 4.1.1.3 or higher to avoid any issues.