Hello, Drew.

No, Cassini on its own does not take nearly as much CPU time. Something is amiss. Have you tried running the app in the WebDev (Visual Studio web server) or IIS? Does every added client make Cassini add 25-35% of CPU utilization?
Please note that "Always keep application in memory to improve first page response time" checkbox on the app registration page makes Cassini request your default page every once in a while, so if the default pages of applications do heavy lifting, then you may see CPU usage go up. In your case it makes sense to keep the checkbox unchecked.
If you wish to profile your app with Cassini to find out where the CPU is getting hogged, try attaching to UltiDevCassinWebServer2a.exe process.

Please let us know what you have found out. We will continue trying to troubleshoot this issue with you.

Best regards,
UltiDev Team.