That could be peak hours, cisco switch as a DHCP server and at the same time it can act as Layer 3 not a good idea.
check how is Switch CPU at the time of Peak,. check the Logs in the Switch any DHCP errors ?
what kind of user base we considering here ?
how about later peak hours, is that get quicker than expected or still delay all time?
check some troubleshooting tips :
https://www.cisco.com/c/en/us/support/docs/ip/dynamic-address-allocation-resolution/27470-100.html
Also, capture from one of the Wireless clients (on controller what causing the delay ?)