The almost bezel-less Mi Mix and the Mi4c from Xiaomi are now getting official support for LineageOS 14.1 ROM. The Mi Mix is of course, the more favorable of the two and it shall look great running on Lineage OS.
With the Snapdragon 821 SoC, Mi Mix was already a powerful chipset so you’re technically not going to see any real world difference in performance with Lineage OS. But coming from MIUI, it’ll be a completely different experience to use an AOSP based ROM. As for the Mi4c, the Snapdragon 808 SoC inside coupled with 3GB of RAM is still an excellent combo, and like Mi Mix, Mi4c too will a perfect match with LineageOS 14.1.
LineageOS is essentially a gateway to a whole new world of customization and performance gains both of which were originally limited through MIUI. I’m not saying that MIUI is bad (in fact, it has many useful features), but LineageOS will basically bring that pure-stock Android look to your device, which also means no bloatware.
To install LineageOS on your Mi Mix or Mi 4c, you will need to have your bootloader unlocked. After which you will also need to have TWRP (recommended) or any other custom recovery installed. The only hassle with Xiaomi devices is that you will need Xiaomi’s permission to get it officially unlocked.
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IF you run into any trouble, you can always ask for help over at Lineage OS community.