When preparing a Flex project for deployment, always make sure to export a release build. I think most people know about this feature of FlexBuilder, but I’m not sure everyone is aware of just how important it is. The SWF that is generated when you run or debug your project contains a lot of extra byte code used for debugging and profiling your content.
This extra byte code has a very obvious effect on the size of your SWF (adding over 35% in some cases). It also has a less immediately obvious (but no less significant) effect on the performance of your application. In one example here at gskinner.com, a 980kb SWF was reduced to 675kb, and some performance intensive code ran over 2.5x faster in the release build than in the debug version.
Never deploy a project without exporting a release build, and be sure to test a release build if you are experiencing problems with file size, performance, or memory utilization.