This is available only in IE 8 (and eventually IE 9). If IE is taking too long to start up, one of the things you can do to troubleshoot this is to go into Manage Add-ons from Tools > Internet Options > Programs and check the Load time column for the various toolbars and extensions:
Since writing this, IE9 Beta has been released and it will automatically inform you when an add-on takes too long to load. This is known as the Add-on Performance Advisor. The threshold has been set to 0.2 seconds. Anything taking longer and you will be advised by IE. Read here for further details.