Make sure you are on the latest Tiger 10.4.6 iMovie 5.0.2 version 3 or 4 not recommended.
If still running Panther make sure you have the latest version 10.3.9
Updating is recommended for all users. Use the Software Update application or download from here.
Apple added a new iMovie HD forum
to deal with new and upcoming issues.
Fixed Problems and Known Issues:
Notes and Hints:
IF ALL ELSE FAILS and iMovie is still launching slow
Update Your Prebindings by:
- Open up Terminal (Applications/Utilities)
- Type: sudo update_prebinding -root /
- Press return
- Enter your Admin password when prompted, then press Return. Wait... (you will see text-galore scroll by for about 10-20 minutes)
- Quit Terminal when the task is complete.