I'm having the same issue at times but it seems random. Some days I boot up, open FCPX and nothing has to be re-rendered, and other days a full hour or more of re-render time is required.....usually it's just small clips but nerve racking when working under a deadline and you think you are done to say the least.
I'm not using raids, just firewire drives that are not disconnected when shutting down. This happened a few times when I was NOT copying media to the events folder, just referencing it from the original location.
More recently it happened in a project where I used the "copy the media to the event folder" option, however it happened after simply opening another project in the timeline, then going back to the original project in the timeline. Nothing was changed in either project while open and FCP was never shut down.
Trashing prefs seems to help with slowdown issues but doesn't have any effect on this. From my experience it seems fairly random.
Any updates on this would be appreciated.