Sometimes an iMovie preference can go awry that causes iMovie to fail to operate correctly.
Try deleting/resetting preferences, as often that will cure export issues. To delete/reset preferences 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. 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.
You can also try creating a new project and placing your clip into that one. See if you can export at 2x speed.
-- Rich