iMovie is lagging. Is it because I’m using 4K footage?

Very high resolution videos can cause some lagging, particularly if you have a lower powered processor, like the i5, or low memory. Insufficient application space on your internal drive can also cause iMovie to function inefficiently. It may also be that you need to reset your System Management Controller (not available for the new M1 Macs).

Check your available disk space (Apple Logo/About This Mac/Storage) and make sure that you have 20-25GB of free space. iMovie can function at 5GB, but you might get some lagging problems. At 2GB you will get a warning message.

To rule out a project specific issue, try creating a new project and sharing out from there.

Try deleting preferences. Open iMovie while holding down the Option and Command keys and select to delete preferences in the box that appears. iMovie will open in a new library. Reopen your old library to get back to your projects.  Deleting preferences is a safe procedure that will not cause data loss or disruption to your project.

Try resetting your System Management Controller, if you have other than an M1 Mac.

https://support.apple.com/en-us/HT201295

Try redownloading iMovie. Move your iMovie app from the applications folder to the trash, but do not empty the trash. Then sign in to the app store and redownload from the display of your purchases. If not successful, move your iMovie app back to the applications folder.

To rule out a hardware problem, run the Apple Diagnostics program on your Mac.

https://support.apple.com/en-us/HT202731

If none of the above resolve your issue, contact Apple Support.  The techs there have the capability of remotely accessing your computer that we cannot do here.  To contact apple support click on the Contact Support item in the upper right hand corner of this forum’s screen.  Navigate to a place where you can enter your phone number.

-- Rich