Why so long to open - "optimizing - processing event"

Running iMovie '08, app installed on my main drive, then my video files are on an external drive.

Each time I open iMovie, I can't do anything and computer kind of grinds to a half while iMovie says "Optimizing Movies"and "Processing Event".

The result is that each time I open iMovie it takes a couple of minutes before I can do anything with the application. Arghh

Is there some preference setting that I am overlooking or something else that is causing this? Or because I have my video files on a different drive?

(using Mac Pro, 7200 speed drives).

Thank you everyone-