One of the improvements in IE 8 was the introduction of a JScript profiler which can provide useful JScript-related performance data during web development, and this feature has also been carried over into IE9.
To access the profiler, from IE9, launch the developer tools window either by selecting ‘Tools>F12 developer tools’ from the menu (shortcut keys: ALT+T, L) or toggle on/off simply by pressing F12.
From the developer tools window, switch to the ‘Profiler’ tab, and then click ‘Start profiling’ to begin a profiling session. Click ‘Stop profiling’ to end the session. You can then view the resulting performance report in either ‘Functions’ view or ‘Call tree’ view, using the ‘Current View’ dropdown at the top of the window.
The developer window has a number of other handy features as well, such as disabling the pop-up blocker, disabling CSS, or validating HTML & CSS, etc.
More information can be found on the MSDN blog at: http://blogs.msdn.com/b/ie/archive/2008/09/11/introducing-the-ie8-developer-tools-jscript-profiler.aspx