Delete the Useless Files and Applications
Like most other electronic devices, more of your memory filled with programs and files, the operating system will be forced. So; You can speed up your device by deleting useless, unnecessary applications, photos and videos. Simply, before deleting, don’t forget to move the ones you don’t want to lose to another computer or storage device. This extra memory space would not only speed up your phone, would also extend your battery life.
Close Background Application Refresh
The function of Background Application Refresh, as the name implies, allows applications to refresh itself when not in use. So, when it is open, it puts an extra strain on the operating system of your device. Although yet, this function keeps the applications up-to-date, leaving it off might speed up your phone. You can turn self-refresh function off for applications all together by following Settings > General > Background Application Refresh, or if you wish you can also turn it off in application basis one by one.
Turn Down Visual Effects and Animations
If your old iPhone is having hard time running latest version of iOS, modify some visual properties will be useful. Open Settings, go to General, choose Accessibility. At this menu, inside of Increase Contrast, turn on Reduce Transparency.
In Accessibility, turning on Reduce Motion will work as well. When you exit Settings, there will be no fluid menu transitions and transparency that you are accustomed to; but it will speed up your device when you turn it off because these animations load your phone's processor. In the latest models, this process will not have much effect.
Last Resort: Return to Factory Settings
As with all Android-based devices, if all these suggestions do not help you, then all you have to do is simply restore your phone to the factory settings and delete everything altogether. However, all your information and applications will be deleted when you do so, so make sure you back up your phone with iCloud or another service. Reset function is at the bottom of the General section of Settings.Once you've done this, you'll be able to log in to your iCloud account and restore your backed up apps and settings as in a brand new phone.
