Hello Andre,
there are several features you can use to load balance the clients between access points in range of the clients:
1. Band steering push clients with 802.11a/n(/ac) to this ap radio module instead of establishing a client connection at the 802.11b/g/n radio module
2. Enable load sharing of clients between access points (possible at site configuration i think) for better density balance
Other reasons for poor wireless performance resulting in sharing the available channels with other access point around the location. In the 2,4 GHz frequency space high channel contention (co-channel contention) is a big problem. 5 GHz is much better, because you have more channels available, but not every mobile support it.
Another recommended practice is create smaller cell sizes and use more access points. In some cases is make sense to only use 20 MHz channels on the 802.11a/n radios.
Best Regards
Hartmut