After you've created and saved the script, open the AVS file with VirtualDub. Using a lossless codec for video compression - I recommend Lagarith - save the video as an AVI file. This will take hours but you only have to do it once.
Now run your MPEG-2 encoder, and select this AVI file as your source. You'll find it speeds along for both passes, because the heavy AviSynth processing has already been done, the CPU is only being used for decoding/encoding.