Reducing flv CPU usage

Hi, I am making a websites with fairly large videos in the background.
The site is extremely heavy for the cpu and I was wondering if any of you guys know a trick on how to make flv streaming less heavy.

Here are all my specs :
Large flv (920 x460), with alpha channel (transparent background), Max data rate:300kbs,
codec : On2 VP6 encode alpha channel, framerate : same as source.

Using progressive download
Video plays in a flvplayback instance nested in a swf that is loaded and added to the stage by an other swf.

Embed in the html page with swfobject, wmode = opaque.
Movie playing at 31 fps.

You got any clues?