Since the iPhone doesn't offer the same flexibility when it comes to system performance like Android does, there isn't much to do it when it comes to optimizing its speed. The only thing you can do is to restore it to the defaults using the Restore button in iTunes. This will restore the iPhone to the factory condition. Additionally, make sure to use only the applications you need. If you install many apps and these apps have background running services, then the performance will gradually decrease with each new installation.