~35 to 50 clients@WAP AC: slow authentication

Hello,

I have WAP ACs running with a CAPSMAN controller running on VMware. (latest 6.40rc32)
When there´s hardly anyone connected at the access point, connecting to the wifi access point is fast. Within <<1 second the client is connected and received its IP address.
This changes when there are more clients concurrently connected at the access point. Then, it can take up to ~15 seconds till the client connects and has its IP. Withing that time frame, the clients Shows that it tries to connect/authenticate until it receives it IP.

There´s hardly any traffic on the access point (5 MB/s), CPU is roughly from 7 to 10%. CAPSMAN controller has nothing to do, either. latency between WAP AC and controller is <1ms, between controller and radius server also that low.

Has anyone experienced this behavior, too?