I’ve been experimenting with timelapse videos and ran into an FPS limitation with iMovie. When importing images into your video project, the minimum amount of time it lets you show each frame is 1/10th of a second. That means the timelapse will play at 10fps, which isn’t ideal when the rest of your video is usually 30fps.
To get the video playing at 30fps (or more) instead of 10, you’ll need to speed it up. Start by creating a new project with only your timelapse images, and setting each frame to 1 second. Then, export the movie at 1080p into a temporary file. Once that is done, import that file into your main project and set the clip speed to 300%. Now you’ve got your timelapse images playing at 30 frames per second.