The version of iMovie I’m using with this machine — iMovie 10.1.10 — has become incredibly slow when rendering 720 videos

Try deleting preferences by restarting your computer and opening iMovie while holding down the Option and Command keys and selecting 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. Now see if the issue is solved.

If that doesn't work, try booting up in Safe Mode (see help menu for instructions) and, after everything loads, immediately shut down and reboot in normal mode. Sometimes that clears out conflicts.

To rule out a conflict with your main user account, you also can try opening iMovie in a new user account such as the Guest account on your Mac, or another user account. Then see if the issue is resolved.

Finally, if you share out in Best Quality (pro res) setting, your file size will be 4x larger and thus slower to render. Also, confirm that the "Add to Theater" box is not checked when sharing out. Sharing to Theater greatly increases rendering time because Theater uses multiple formats to make the movie playable on various devices.

-- Rich

I would repost your question in a new thread, refer to this thread and tell what helped. That would re-expose your issue to the entire community for further comment. If the issue is prevalent perhaps someone has found a permanent solution. I take it that you have tried Safe Mode and opening in a new user account as I mentioned in my earlier post.

-- Rich

Oh and by the way....

If you cancel the export, do you get a message saying that the export was successful? I am having the same issue as well....

https://discussions.apple.com/thread/250117973

Somewhat,...

The export freezes when the circle thingy reaches the halfway point.

Great. Glad it worked for you. :)

-- Rich