DefaultItemRenderer & ItemRenderer

Yes, performance. While it's not a huge difference, it was important enough for us to make the DefaultItemRenderer we ship with the Flex framework as fast as possible.  Because of this, we went with a custom AS class that had layout baked in for our simple text item renderer.  http://opensource.adobe.com/wiki/display/flexsdk/Item+Renderer+Changes will help give this decision context.

We still reccomend people use an MXML ItemRenderer most of the time; however, recognize that dropping in to ActionScript code and/or extending UIComponent instead of ItemRenderer will shave some time off for those that are performance-worried.

Hope that helps,

Ryan