MIUI Optimization objašnjenje sa ostalih stranica:
• Turn On MIUI Optimization : This option enables a pleothora of MIUI based settings & optimizations as per the guidelines set by the MIUI developers. This setting can sometimes cause a variety of issues on non-MIUI based apps such as Google Apps & Apps from Google Playstore. Its better to disable if you rely on Google Playstore for apps & services.
With the MIUI Optimization Enabled, some users reported the following issues - Cannot set third-party launcher like Nova, Apex or Google Now launcher.
- Wallpaper cannot be set using in-built options via custom launchers.
- Occasional lags, stutters or freezes while scrolling web-pages & long lists.
- Custom apps set as default restore to default MIUI apps on reboot. (Only on MIUI China ROMs)
- Cannot set accessibility services on reboot.
- Background apps cannot sync data.
- Music players stop playing after sometime.
- UI Animations are not synced properly.
- the battery usage report become the stock aosp android.... BUT you can still continue to enter the old miui battery report, entering in power menu, and battery stats.... in that way you return to the miui hardware/software report.
so imho..... it's GREAT to have all 2 types of report, having miui optimization OFF
(
miui battery stats page is still there. you find it at battery>battery power>battery use .............. instead to open directly the battery>battery use menu 
)
2) i've noticed that the bad quality screenshot is only with HARDWARE BUTTONS. can you confirm?
usign the software button way, quality is perfect, just using power + vol down pictures are worse.
BUT I MUST ADMIT THAT i never used hardware method while the miui optimization was ON... so i should test it now to be sure that returning stock/ON the quality of screenshot will return perfect also hardware way. (maybe it was bugged also with miui optimization ON... just to say)
After turning off MIUI optimization - Above issues may get solved.
- Settings → Data Usage → Graph changes to AOSP style.
- Settings → Battery & Performance → Battery Use → Graph changes to AOSP style.
- Settings → Installed Apps → All → Documents → It will not be disabled after every reboot.
- Settings → Additional Settings → Buttons → Settings may get reset.
- No OTA update notifications. Enable it to receive the OTA updates.