Low performance while encoding

While investigating live HD broadcasting using new thunselda builds, I've noticed that ffmpeg2theora's performance is less than satisfactory. The input format, resolution, framerate or scaling do not seem to play a role - the main factor seems to be output bitrate - if the bitrate is higher than aprox. 1Mbps, ffmpeg2theora doesn't seem to be able to stream it's output in realtime. Moreover, using a faster machine does not produce better results: on a Intel Core 2 Quad Q6600, average CPU usage is around 30%, and encoding speed is still less than real-time.