Sometimes my Mi Box S was slow, so I tried changing HDMI Cec options because it was when my tv turned on the moment the mi box was slow -after sleep stand by-. I can't remember what I did but dealing with these settings (hdmi cec and stand by) solved my problem. Good luck.
I would use SD MAID from apk time and it will free up space and your MI box should run a lot smoother
In general disable any apps you don't use as mention. But before you disable them uninstall any updates then disable them this will recover a lot of storage. Try to make sure you have at least 800 MB space free as some these updates are getting large.
Also try using the device without Ethernet USB dongle to eliminate that as a issue unplug it from USB port.
If the factory reset doesn't work return it get a replacement.
Default android advice is, uninstall the apps you don't use and make space.