Isn't it the combined effects of HD resolution + the many motion vector calculations being done by the script?
Try running the script in VirtualDub, hit Enter and view the stats - what sort of playback FPS do you get?
FWIW, on my ~3.8ghz i7 machine, encoding a grainy BD source (from a file captured from HDCAM SR tape) chugs along at 0.6fps during the very grainy scenes, on the veryslow preset. The quality makes it all worthwhile though
