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Delete temp. files / Clear the cache
Maybe the temp files and the cache are slowing down your mobile device. Temp files are created on a daily basis and a few of them don't pose a threat. But, too much temp files can definitely slow it down.
Multi-tasking does make it easy to switch between different tools, but as a result, the device's performance is decreased, due to lack of free RAM. To fix this (and to free up RAM), consider closing all the applications you don't use.
Every application installed in your mobile device runs in the background and slows it down, no matter whether you use the application or not. The best way to prevent applications from slowing down your mobile device is to delete them permanently.
The operating system requires a certain amount of free space (at least 1 GB) in order to function properly. If it lacks free space, the device becomes slow. A large number of personal data files such as photos, videos, music, applications, etc. can significantly slow down your mobile device.
Maybe your device is slow because of a virus, which you have picked up while browsing the net or by installing insecure, modified applications, downloaded from third-party websites other than Google Play(store) or App Store.
System updates can improve the performance of your mobile device. That is why it is very important to check for updates frequently. Usually, your mobile device checks for updates automatically; however, it would be better for you to check for updates manually.
The GPS feature can also slow down your device because it performs constant scans to update your location. If you don't need it, you can disable it to improve the performance (and the battery life) of your mobile device.