Mike Taulty decided to revisit his “whimsical” sample that he did earlier in the year and see how it worked in IE9 with windowless mode.
Source: Mike Taulty's Blog
If you search the web for something like “Silverlight windowless performance” you’ll find a bunch of articles that talk about how you want to avoid using Silverlight in Windowless mode in the browser for performance reasons.
The main reason why you’d use windowless mode in the first place is when you want to overlay Silverlight content on top of HTML content and, usually, there’s a performance penalty to pay for this because the browser has to take over the rendering and do the work of blending the Silverlight content with the HTML content that might reside “beneath it”.