One of my customers faced a problem that suddenly their SharePoint portal was loading extremely slower than usual. After some basic troubleshooting I did not find anything suspicious in the ULS logs, IIS logs or even Event logs.
After that, I came to the part that I like most which is capturing a memory dump for the IIS process and analyzing the threads running. I searched for any common mistakes like looping a large list, calling a remote web service but couldn't find any.
After a deep analysis of the memory dump (Which was done by an Escalation Engineer for SharePoint), it seems that the farm root certificate was missing and therefore was trying to validate it from the internet every time the user requests to load the page and this was the resolution http://support.microsoft.com/kb/2625048