When you take a project that was started on PPC and then opened on an Intel Mac any rendering of effects will take longer than a project you start brand new on the Intel Mac.
I have a project created on my iMac G4. I open this on my new C2D iMac. I take a 1 minute clip and apply the Aged Film effect. It takes nearly 2 minutes to render the effect.
I then create a new project on the C2D iMac. I import the same 1 minute footage (full quality DV). I apply the Aged Fillm effect. It takes 55 seconds to render.
Can anyone else confirm this behavior? Any clues? Is iMovie being moved into Rosetta mode when you open a project that was created on PPC?