Hi, I have a MiBox since a year and until yesterday works great. Now I'm trying to use with a 4K tv with poor results.
Kodi has many of lags while playing videos, even if the played video isn't 4K and/or h265.
Basically, the same video is laggy on a 4K, but it looks good on a FHD tv.
Make sure your box is up to date as there have been optimisations within the last 6 months, and there will be more soon. If you're on the Nougat beta, there is a means to back up your DRM and bootloader so you can downgrade again without losing anything.
The Mi Box cannot do above 4k30fps with x264, but it can reach 60fps with HEVC/x265, at least in terms of the chipset.
I'm sure there's UI optimisations you can make in Kodi to help with any slow downs there, but also test how YouTube handles video (again, make sure the app is up to date).