I'm trying to use the mirror monitor function for the purpose of showing a video on a difference monitor, but the default update rate is SO SLOW that it's completely unwatchable. Is there a way to alter the update rate?

Try mirroring the video player window rather than the whole monitor.

I'm mirroring the VLC window to a second monitor and it's running smoothly. Like you, when I mirrored the monitor, the playback was choppy/low FPS