Open Task Manager, find the IIS Worker Process that takes a lot of CPU
Right click that process and select Go to details:
Check who is the user running that process under User name column (it is "AccessPortal" in the following sample screenshot):
Go to IIS Manager, Application Pools, find the pool with that name, right click on it and select View Applications:
Check what is the name of Site that runs on that application pool (it is "AccessPortalBastion" in the following sample screenshot):
Open sites list in IIS manager (by clicking on Sites folder in the left panel) and check what is the id of the needed site (it is "2" in the following sample screenshot):
Last step: go to C:\Inetpub\logs\LogFiles and select folder W3SVCN where N is the id found in previous step (like W3SVC2). Copy log file from the date the IIS was low and file from the date it is high.
Thank you!