This is often found with customers who have wireless 802.1X (sending RADIUS requests from their APs/WLCs to ISE). Have a look at the timeout settings on the AP/WLC - if it's 2sec, then change to 5 sec (Cisco recommended value).
I have also seen this a lot with users roaming between APs - each time a user roams, it causes another 802.1X request - if the WLC is not caching the keying material, then it causes a lot of traffic to the RADIUS server.