From what I could tell when I reverse engineered the firmware, it is nothing to do with performance but the GUI. It replaces some AOSP-style themes and such with MIUI redesigns and branding. Having it on gives an MIUI look in other apps, but turning it off returns a stock Google-look in other apps. It will make your device more compatible if you leave it off.
I could be wrong, Xiaomi is changing all the time and adding weird/secret things to firmware, but when I checked a few months ago this is what it seemed to do. Won't help performance or battery life either way.