[There are specific instructions available for Android and iOS](/learn/why-is-phone-running-slow/#easy-ways-to-speed-up-your-phone), but a quick look at an app's settings should display how much data it's using. If you have apps you no longer use, delete them to free up storage and reduce the load on your battery; this may help speed things up, and reduce the bandwidth your phone is using. If you see that certain apps are using lots of data, explore the settings for that particular app to find ways to restrict its data usage.