You may have installed some software on it that use too much CPU or GPU or RAM or space. There are some apps that are working in the background with out you know it. You must either close them down or delete them. Facebook messenger and Skype and other social apps are known to bring an iPhone to its knees due to bad programming. Uninstalling some of the CPU/GPU intensive apps may help your iPhone from lagging. If that won’t work either do an “erase all content and preferences” with out recreate everything from backup. This method will help 99% of the times because your problem could be a mix of unfortunate settings that is messing up your iPhone.

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