When I'm making projects with Flex (or Flash Builer) the end result usually performs much better than a similar level project made with the Flash authoring tool. I know Flash IDE is rather for animations and design heavy works so you can't really compare these 2 tools, but this question is bugging me for a long time.
If apps made with Flex (or Flash Builder) are really faster and less CPU intensive - why is that?
Depends how you write the app, it has little to do with the tool you use. For example, you could write a pure AS app using Flash IDE (which is a bad idea as there are so many other awesome tools that can help you do that, beside the Flash crappy editor) which will be smaller(KB) and faster then a similar Flex app because the flex app will have a lot of redundant code behind (the entire framework). However if you choose not to write pure AS and use the graphic editor (dragging shapes, animating using frames) you would add even more redundant code to your app than Flex would, and that would make it slower than the Flex app.
In short terms, if you want speed as in development speed (how long a project takes) use Flex, it has lots of pre built things that help you put up your project fast. However, if you want speed as in, performance and small size, use pure AS with a nice editor like FlashDevelop. It's a matter of trading performance for developing time I guess. Never use Flash IDE graphic interface for neither :)
