Have you tried playing with frame/refresh rate settings across your TV, Mi Box and Plex/Kodi? I can't speak for Kodi, but I know Plex has a 'refresh rate autoswitching' that can be turned off/on.
My guess is that you could be sending a 24 fps video to a monitor locked at 60/59.9 fps. That can cause the studder like you're seeing because you can't divide 24 frames of the video file evenly across 59.9 frames on your TV. Look at the MediaInfo for the video file if you're unsure or want to confirm.
Hope that makes sense and gets you in the right direction. I'd try the Plex settings first, turn ON refresh rate switching. Then try to make sure your TV isn't hard set to 60fps...look for 'auto'. I'm assuming Kodi has a similar options as Plex.
Last thing you can try is different audio tracks if the movie has them. Sometimes muxing one format to another (ie. Playing Dolby Atmos with a non Atmos receiver) can cause studder for similar reasons as above.
@/u/Shardrock
Great detective work....
/s/ I will be sure to pass this solution to the Xiaomi dev team so they can be sure to reach out to connected TVs with this problem and teach the MiBox how to interface correctly to the myriad of ancient TV makes and models that are used with MiBox devices. /s/
I had a similar issue and ended up returning it. Shield TV has been on sale lately!
Sorry guys I'm not sure how/if I can mark this as solved...
In what is admittedly the worse troubleshooting know to man, I have fixed it (sorry shield guys, not today!).
The TV it's hooked up to (straight connection) is a Samsung UN55D6000 (cira 2011) and these are the steps I took - all at once 😬
Playback on native plex app and Kodi plug-in is buttery smooth at last! I may turn these settings on one by one so I can update this to be more useful.