Hi,
You Mac specs are perfectly adequate for editing movies.
There can be several causes for choppy video. Sometimes a difference in frame rates between the clip and the frame rate of the project could cause choppiness. For example, playing a 30 fps clip in a 60 fps project. Or possibly the clips have a variable frame rate rather than a constant one. Or possibly the clips are in a format not fully compatible with iMovie, or maybe have audio/sync issues. You might find this link helpful as to the causes and solutions for choppy video.
https://www.quora.com/How-do-I-fix-iMovie-making-my-video-editing-choppy-playback
I would try these three diagnostic procedures and see of either of them solves it.
Try deleting preferences. To do that, open iMovie while holding down the Option and Command keys and select to delete preferences in the box that appears. iMovie usually will open in a new empty library. If so, reopen your old library to get back to your projects. Now see if the clips play smoothly. Possibly you may need to reboot to set the changes. Deleting preferences is a safe procedure that will not cause data loss or disruption to your project.
Try placing the video clips on the desktop and re-rendering them. You can do that by Control-clicking on the clip and selecting Encode Selected Video Clips from the dropdown menu. It will create a new movie in addition to your original clip. See if the re-encoded clip plays smoothly in iMovie.
If still no success, try using the free download, Handbrake, to convert the clips to Mp4/AAC, that Handbrake will do by default. In the settings, choose Constant Frame Rate. The conversion is to correct any format, audio/sync issues, frame rate and rendering issues. Then import the converted clips into iMovie and see if they play.
You can get Handbrake here:
https://handbrake.fr/
A simple way to do it is to open Handbrake and do a File/Open Source. Navigate to your video and choose it as the source from the resulting screen. Then do File/Start Encoding. Wait a couple of minutes for the conversion to complete. Then save and import the converted clip into iMovie.
-- Rich
