As you may know you can set the EnableFrameRateCounter="true" property of the <asp:Silverlight> control to display the actual Silverlight framerate in the status bar of the browser.
(You can also set the enableFramerateCounter param if you use the <object> tag or even enable it by code:
Application.Current.Host.Settings.EnableFrameRateCounter = true;)
But if nothing appears in your IE status bar you may have to change some security settings:
In IE go to: Tools / Options / Security
Select “Internet” zone and “Custom level…”
In the “Settings” window go to the “Scripting” section and enable “Allow status bar updates via script”.