It is not the Flash 9 Development Environment that makes the code 10x faster, it is the (free) Flash 9 Player. That means even when you keep on using Flash 8 for development, you can benefit from the increased performance when the .swf file is run on a Flash 9 Player.
What's really new in Flash 9 is ActionScript 3, which now is a 100% Ecmascript compatible and very sophisticated language and more mature and feature-rich than ActionScript 2. However, I think that this is more important for Adobe Flex 2 developers than for Flashers.
ActionScript 3 also comes with a more advanced framework. For example, it now also supports binary sockets and not only XML sockets.
Another advantage for Inel Mac-users is that the environment is now a Universal Binary. Windows-users won't feel that difference. ;-)
What's really new in Flash 9 is ActionScript 3, which now is a 100% Ecmascript compatible and very sophisticated language and more mature and feature-rich than ActionScript 2. However, I think that this is more important for Adobe Flex 2 developers than for Flashers.
ActionScript 3 also comes with a more advanced framework. For example, it now also supports binary sockets and not only XML sockets.
Another advantage for Inel Mac-users is that the environment is now a Universal Binary. Windows-users won't feel that difference. ;-)