Cisco Wireless :: AP 541N With Clustering Function

We have got 3 Cisco APs 541N with firmware version 9.2.2 and want to set up the cluster function for those APs.Here is the problem:Before enabling the cluster you can ping all three APs located in the same local network with a delay of 1ms or below.If the cluster is enabled the pings for one AP rise above 200 ms in a random way (see attached file).Moreover users connected to that AP are more likely to lose the connection or experience delay.Do the APs communicate through LAN or WLAN ?