In the HierarchialBrowser I am working on and will be talking about at 360 Flex I was noticing a performance problem and found out that the culprit was setting a large list with over 40,000 item selectedItem to a string.  Apparentely this causes a long search across the list.  I changed it list.selectedIndex = 0 and that reduced the time down to about 10ms.  The gotcha is to use selectedIndex when possible on a large list and lookout out for cases when you failed to do so.