Choppy Playback in iMovie 10.1.14

It could be that your clips are shot in a lower frame rate than your project. Take a look at this thread for a list of things that can cause choppiness in playback:

https://discussions.apple.com/thread/251227411

-- Rich

You can check the frame rate of your project by opening the project in timeline view and then expanding out your time line by using the slider next to settings button above the timeline on the right hand side of your screen. Then put your cursor on a clip and, while holding down the "r" key press-drag the cursor slowly to the right. A yellow frame and frame counter will appear and record the frames as you drag. If it is a 30fps project the frame counter will reach 00:29 frames and on the next tick will read 01:01 to indicate one second, thus indicating a 30fps project.

It might be helpful to review the procedure for setting frame rates for iMovie 10. To set the frame rate of a project you must create a totally new project and then place a video clip (not a photo) with a known frame rate as the first clip placed into the project. So, if you put in a 30 fps clip as the first clip placed into the project, the project will be set at 30 fps. You can then delete the first clip without affecting the project. The project will export (share out) at 30 fps even if the subsequent clips that you place in the project are other than 30 fps. If you erase all the clips in the project and start over with clips of, say, 60 fps, the project will still remain 30 fps because that was the frame rate of the first clip placed into it when the project was newly created.

-- Rich

Yes, that means that the project is 30 fps. But if the clip was, say, 12fps before importing it into the project, then playing it in a 30 fps project might cause choppiness.

— Rich