This is a place where the Swfdec developers dump answers they give to frequently asked questions. So we chose the obvious name FAQ for this.
What do I have to do to make video playback in Swfdec as fast as with mplayer?
The short answer: It's hard. Here's the problem: As you might know, hardware has a dedicated method to display video, called the video overlay. That is what xv and in turn mplayer and ffmpeg use. It has the following features: * reserve a rectangular region on the screen for video display * move a memory rectangular image of YUV video data to that region and scale it to fit. That's not a lot and works well enough for video, but not for Flash. Flash allows rendering stuff on top of the video (the end screen on Youtube for example has the last video image shine through) and it allows translucent videos and drawing non-rectangular parts of videos. All of this is not supported by xv, which is why we decided to not go through the pain to use it. The unfortunate side effect is that currently a lot more horsepower is required to display a video via Swfdec.
The end goal is to use OpenGL and its video extensions to speed up Flash video. That should make it as fast as xv for graphic cards that provide these features (almost all current graphic cards do). But then, there is currently no OpenGL cairo backend, even though there's constantly talk about doing one.
Old versions of swfdec (0.6.x) didn't automatically start flash applets, new versions have autoplay. How can I get the old behaviour back?
The Feature is called autoplay. When right-clicking on any flash applet, you'll get a Menu entry "Autoplay". Set it to "Never" to get the old behaviour back.