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☞ [Tips & Tutorials] Phone Getting Slow? 7 Tips to Speed Up Your MIUI!
Many complained that their phones are getting slower and laggier after a period of time, but this is common among Android devices. People tend to reboot their phones to get it running fast again, but there's a few things you can do before you reboot, to ensure a smoother experience after the boot up.
1. Update to the latest version of MIUI
Updates will carry fixes for previous bugs and some optimization for the phone that'll increase performance.
2. Clean app cache and system trash files
Data and image caches usually builds up after users access some apps or connect to a network, these files are the culprit causing the phone to slow down. It's recommended that users clear out cache of apps that aren't used for long periods of time in Settings > Installed Apps > Select any app > Clear cache. In Security, choose Cleaner and let the scan run then clear out the system trash.
3. Disable or reduce system animation scale
In Settings > About Phone, tap on MIUI ROM version until it shows that developer options are enabled. Then head to Settings > Additional Settings > Developer Options > then alter the system animation scale.
4. Restrict background app and autostart
Also in Developer Options is the option to limit or disable background apps to reserve enough memory to ensure smooth performance. In Security, we can also disable apps from autostarting to shorten boot up time and save on RAM.
5. Performance mode and system optimization
In recent MIUI 6 updates, a new Optimize system performance is added. In Settings > Additional Settings > Battery, change your option for system optimization. If you wish for a snappy and smooth experience, you'll have to turn on Performance mode here.
6. Force stop or uninstall apps
Smartphones nowadays are usually common with heating and battery draining problems caused by apps. Users can check the battery consumption of apps and after determining the app that consumes the most resources and slows down the phone, stop the app from running anymore. Some times, apps that were not used can bog down the phone and consume system resources, thus it's recommended to remove the apps.
7. Factory reset or fastboot flash.
Some times, it's necessary that users factory reset their phone, especially when your phone is still running ludicrously slow after following the above steps. By clearing out your phone of all data, apps, images, audio et cetera, your phone will become "brand new" again after a factory reset. Do remember to make a back up of everything. Fastboot flashing via MI PC Suite will also remove all files in your phone. These processes will help to remove apps and files that causes the system to slow down.

☞ Mi 4i battery doesn't charge beyond 35-40%
1. Drain battery below 15-25%
2. Switch off the phone
3. connect the charger and keep it for 4-6 hours or more
4. Remove charger and turn on the phone it will be charged only till 35-40%
If you keep the phone ON and then again connect your charger. It will charge smoothly up to 100%.

Semoga bermampaat.