I hope this is helpfull, With anything if your drive is at 90% full performace will suffer. Also if you are working from a sequence timeline of 90 minutes kulled from 40 hours worth all still available on the drive spells seektime trouble. I would separate the wheat from the chaff to reduce seeking. Its not and FCP thing as much a OS, IO, filesystem, drive speed issue. If you can, I would buy a new FW drive and tranfer onto it only the stuff you know you want to continue working with, say 20 of best hours of footage 🙂 and leave the remaineder on the original drive for quick access in case inspiration hits and the footage wasn’t transfered over. That way you have reduced load on the system and added a back up of the project footage.