I've worked on Flex applications for the past 4-5 years and see a lot of developers picking it up since it is easy to create rich applications. However performance is often an issue.
I mostly see it when using a lot of styles (or one large CSS file) and skinned components (It is even worse with Flex 4 then it was with Flex 3). When a Flex application gets really large the UI is blocked because there is too much actionscript code needed to get things running. (with this I mean the processing time is acceptable but UI is blocked so the perception is that things are slow) Therefore I'd like to vote on improving the performance of the Flex framework where possible so new and existing applications can benefit. Flex 4 with spark is great but comes with some performance drawbacks, I hope we can improve on this significantly. I'm speaking on behalf of the experience and perception in the company I work for, I'm curious to see if this is also a problem for the rest of you. I'm not the expert here but I'd like to get involved and learn so I can eventually help to fix issues but I believe UIComponent had some overhead and this together with the StyleManager can cause performance drawbacks in large applications
