For the best performance, try to keep at least 1 GB of free space. If the available storage is always less than 1 GB, your device may slow down, as iOS or iPadOS repeatedly leave room for more content. There are several possible reasons why your iPhone may be running slow and feeling sluggish. These can include outdated software, insufficient storage space, background applications that consume too many system resources, and more.
Fortunately, Apple has provided a variety of simple tips on its official support page that can help you fix these performance issues and improve the speed of your device. Chances are that one of the main causes of your iPhone becoming slower is iOS location services. There are some apps that need to know your location, after all, how is Google Maps going to give you directions if it has no idea where you are? But there are many more apps that don't need to know your location or that can be changed to “while using the app”. This continues to affect iPhone 6, 6S, SE and 7 models, as well as iPhone 8, X and XS models with iOS 12.1 or higher.
Check the battery status of your iPhone in the settings: To ensure the maximum performance of your iPhone, it is important to monitor the battery status. While the iPhone's low power mode can be useful for saving battery, it can also slow down the iPhone. In other words, a slow-running iPhone, especially a slow iPhone 6, is a known problem, which is due to Apple's own “performance management changes”. Restoring your iPhone to its factory settings because it's slow should be your last resort, as it erases all the data from your iPhone.