iMovie Lagging on MacBook Pro 14in

I just got the new MacBook Pro. I am editing very normal video (recored with iPhone7) for a project. I have a few clips with background music, and anytime I make a change it starts lagging super bad to where I have to wait a little. I spent a lot of money on this MacBook, so how come I am lagging SO BAD?? I have the 16-core CPU and 14-core GPU (only 1 little upgrade). There is literally nothing on this MacBook; I just got it. There are also no other applications open.