I’ve been working on a large video project and have been frustrated by extremely long Compressor sessions. Taking a one-hour video from FCP 7 edited in ProRes LT to H.264 720p was taking 15 hours or more in Compressor 3.53. I tried a trick (from CreativeCow Forums) to take the file to Compressor 4 by saving the Comp 3 file and reopening it in 4, but it was worse — the time remaining to finish kept growing until I finally canceled it when it grew to 30 hours.
Tried another suggestion from CreativeCow to Reset Background Processing and that helped a lot. Transcode times dropped to 8 hours for a one hour video.