I know that's a big topic. However, it's better to know some general ideas to handle such problems. I have an application which requires Flex to render statistics data calculated instantly on the client side from a downloaded data set. The problems are: the data set is large and needs more than 10 seconds to be downloaded. there are some filters to control the statistics calculation algorithms. If user changes the filters, it would take a long time to recalculate the result and freeze the UI.