From what I understand, "speed tax" will be applied if you use both
Stage3d.http://www.adobe.com/devnet/flashplayer ... tures.html
Who should use premium features?
The premium features tier is intended to benefit game developers interested in creating the most advanced, graphically sophisticated, next-generation games for the web. The combination of hardware-accelerated Stage3D with domain memory enables the use of compiler technologies like the Adobe Alchemy C/C++ compiler, and advanced C/C++ game engines.
Games and applications using either hardware accelerated Stage3D or domain memory individually do NOT
require a premium features license. For example, Flash game developers can independently leverage high-performance Stage3D hardware acceleration in Flash Player – which delivers full GPU-accelerated experiences to more users than any other web technology – at no charge.