This one may seem obvious, but it’s still worth suggesting the tried and true method of “turning it off and turning it back on again.” In other words, resetting your iPhone can be imperative to clearing up any traffic within the device and getting the device back to its normal working condition. The reason a reset can really help with a slow iPhone is that resetting your device can help clear up the device’s RAM. Additionally, resetting your phone to varying degrees can also help clear up any glitches that may have wormed their way into your iPhone.
There are two ways to reset your iPhone. First, you can do a soft reset. On an iPhone X or later, all you have to do is press down on the power button along with one of the volume buttons. This will bring up a reset screen where you can slide the phone to “power off.” Then, wait for about 30 seconds before turning your phone on again. Additionally, you can do more of a “hard reset” and restore your phone to its factory settings. This is done in the “General” section of settings. Once in your settings, click on “Erase All Content & Settings” to restore your phone to its default, factory setup. Before doing either of these, make sure you also back up your phone just in case!