Two words - Developer Dashboard.

Ever had a slow SharePoint site and been unable to figure out what's causing the issue, or where to turn next? Once you've given up on resetting IIS and checking your performance monitors, you'll be happy to find out that Microsoft has a fantastic tool built right into SharePoint 2010 and later.

There is plenty of good information all over the internet about this already, so I will just point to a couple of excellent sources:

http://blogs.technet.com/b/wbaer/archive/2009/11/21/welcome-to-the-developer-dashboard.aspx

http://sharepointyankee.com/2009/12/24/the-sharepoint-2010-developer-dashboard/

Happy bug hunting!