Can you test the back end http service independent of Flex to see how fast it is... Make as many improvements you can, including server side caching if possible.
Firebug plug to Firefox in can give you a quick view of how fast your data is returned and what size the payload is If the data service is returning your data fast enough and Flex is working too hard parsing the data can you pre-aggregate the data on the server side with counts/group by's and return just the data the chart needs to display -Be sure to load test more then just one browser what is you expected user base -Be sure to optimize you query (show plan.. ) add indexes if possable I am sure there are other tricks too but these you can do in the short term
