By very slow, do you mean that it looks like its recording below real time. Or do you mean that the playback is too slow?
If you haven't already, record a short video then play it back. The playback on my machine is much faster than while it is recording. I turned the shadows on and showed a complex area. The animation is painfully slow and choppy and it took about 5 minutes to record 1 minute of video, but the playback looks great!
The playback is very slow because it's a large model so the program is double slow (because of the recording and because of the large of the model). Any idea to make faster?