In this article we'll take a look at a few reasons that iMovie may be running slow and some ways to troubleshoot those issues.
Slow It Down Now
iMovie is not the largest or most complex non-linear video editing program, but that does not mean that it is going to be running quickly at all times. As with any program, if iMovie is running slow then this could be because of outside things happening on the computer or problems with the iMovie software itself.
The first thing you should do when iMovie is running slow is to take a look at your computer as a whole. Start by closing out other programs that may be competing for memory on your computer, especially those used in multimedia post-production. From here you should then start clearing out your Mac, first focusing on getting rid of old files and then defragmenting your hard drives. These are all things that are going to end up having an impact on iMovie performance, as it will with everything on your computer. Storage space is going to be a problem at all times when video editing, partially because of the shear size of video files and the amount of power that a program like iMovie needs to let you edit with real time playback.
If iMovie is running slow during video editing there are a number of very specific things that could cause problems. iMovie is basically designed to handle 16-bit audio, which you can set on your camera when you are actually filming. At times there have been issues in iMovie where 12-bit audio is used, and this can cause iMovie to run slowly.
Likewise, you are also going to have different issues with different audio clips that you may have brought in. If you copy and paste a lot of clips, especially of the same source clip, this has been reported as a cause for iMovie to run slow.
When you are working in iMovie you may just want to limit some of its functionality if it is running slowly. First, you may want to lower the quality in the playback settings of the iMove preferences. You may also want to shrink the overall size of the iMovie window that you are working on. Then go into the iMovie trash and empty it, which is going to free up quite a bit more memory for the program as well.
The formatting for the project disk in iMovie can be the main cause of iMovie running slow. The disk that you are using with the project must be using the Mac OS Extended format, which you may not be running if you have purchased a new FireWire portable hard drive. This is going to be important for iMovie to save and render files. Go into the Finder and right click the disk you are using, selecting the Get Info option. See what the format is, and if it is not Mac OS Extended then you can easily reformat it with Disc Utility.
Third Party Plug Ins
There have been a lot of third party add-ons used, especially QuickTime add ons, and several of these have caused problems with different versions of iMovie. The main culprits are 3ivx, Divx, and Flip4Mac, so if iMovie is running slow and other troubleshooting options do not help then you may want to look into uninstalling these add-ons.