Try some of these tips to improve the time it takes iMovie to open, if iMovie is slow to respond to clicking or dragging, or if video or audio in the playback window is jumpy.
Install the latest software
Periodically, Apple releases updates to iMovie, QuickTime, and the Mac OS. Make sure you've installed the latest versions of this software. The Software Update feature in Mac OS X makes downloading and installing the latest updates a snap. Of course, the latest version of QuickTime is always available from the QuickTime website.
Make sure your computer meets the system requirements
Each version of iMovie has different requirements. Make sure your computer is able to take advantage of iMovie's features.
Rendering clips takes a lot of processing power. If iMovie seems unresponsive, consider waiting until a clip is finished rendering before continuing or rendering another clip. You could also render clips in small batches to improve responsiveness.
Change playback quality
If you are using iMovie 4 or later, consider changing the playback quality setting in iMovie preferences.
Turn off Audio Waveforms
If you are using iMovie HD 5 or later, consider turning off "Show Audio Waveforms" from the View menu to free up processing power.
Don't store your project in a folder that uses FileVault
If you are using Mac OS X 10.3 or later, make sure the project you are working on isn't saved in a folder that's protected by FileVault.
Quit unused applications and users
Quit any open applications you're not using. If Fast User Switching is on, consider logging out other users.
Check the disk
Open Disk First Aid. Check the startup disk, and the disk where the project is saved.
Use Disk Utility to Repair Permissions on the startup disk.
Check for plug-ins
Check to see if there are any plug-ins in this directory: ~/Library/iMovie/. If so, remove them. Reopen iMovie and see if performance is improved. If this resolves the issue, contact the plug-in's developer for more information.
Copy the project to the computer's hard disk
If your project is saved on an external hard drive, try copying the project folder to the computer's hard disk (if there is room) and open it from there. See if this improves performance.
Rebuild the project
If you are only experiencing the issue in one project, consider rebuilding it.
Defragment the disk
If you have access to a disk defragmenting utility, consider defragmenting the drive where the project is located.