Dhiresh Yadav is a wireless expert and working for the Cisco's High Touch Technical Support (HTTS) team, a team that provides reactive technical support to majority of Cisco’s premium customers. In this document Dhiresh has explained different troubleshooting steps if there is a Client throughput/Speed issues in wireless.
User complaints that the Wireless speed is not good and wants us to fix the issue.
Test the user claims by checking the speed/throughput using some tool like Iperf, setting up FTP server on the LAN etc or any other tool. This should tell us the throughput and speed while accessing any internal resources via wireless and will rule out any Internet access related issue as well.
If you still think after above analysis that this might be throughput/speed issue then, try to check the following points
The data rates on the WLC should be set to the rates as per the requirement for example If there are no 802.11b client you can set some higher data rate as mandatory in 2.4 Ghz and disable lower 802.11b data rates. The Management frames like ACK or beacons use lowest mandatory data rates and they eat up most of the airtime (for example if set to 1 Mbps )and if the lowest mandatory rate is itself a high data rate, then it might pull up the throughput of the AP cell. The only thing to consider here is that, disabling lower data rates will reduce cell size and you should be sure that there are enough number of APs for proper coverage.