It's most likely interference in the 2.4GHz frequency band. Where possible it's always best to try and use 5GHz as there is more spectrum available and less chance of interference. Have you tried different channels on the 2.4 radios and/or done a spectrum analysis and checked the SNR of your clients?
Also, looking at the config you've pasted in, just check that TKIP encryption is disabled and use only WPA2 AES CCMP PSK. TKIP can impact performance but won't account for the 2.4/5GHz difference you're seeing.
Hope that helps,