ClearPass has a server configuration option: RADIUS Dynamic Authorization (DM/CoA) Delay which can be set to 0-15 seconds. And there is the Login Delay in the Web login configuration which probably is what you are looking for.
If you can control the switch side, it may be an option to mimic the way Instant AP and controller do a login by posting the guest credentials to the AP/controller and use RADIUS in the back to ClearPass. That takes out the delay caused by CoA; if you don't need to switch VLANs.
Also, if you are a switch vendor and developing products, it may make senses to join the Aruba Technology partner program: https://www.arubanetworks.com/partners/programs/