Imported video running poorly in Final Cut Express viewer

Hi. I'm using a MacBook Pro and Final Cut Express. I'm trying to edit a video, but it is not running smoothly in the viewer so it's impossible to select the in and out for editing. I can't see what I'm doing. Help! Other videos I've imported for this project are running fine. Not the one I imported off of my iPhone 6 in 720pHD.

All the places I looked say to optimize the video but all the explanations of how to do that are for Final Cut Pro and don't appear to apply.

I have 38GB left in storage so that should not be an issue.

Thanks.