If you notice games running slow on Android, here is what you can do. You can clear the cache of android by going into recovery menu and wiping cache (Be careful not to factory reset). You can also make sure that other resource intensive apps are running in the background which will in turn make your game run slow. Android lets you see running apps if you go into settings, tap on Apps and then swipe left to see the running tab.
You can also improve speed by ensuring you have the latest version installed from the Playstore.
People also reported seeing farm Heroes saga running slow, and you can follow same steps above to try and fix it too.
For iPhone follow this guide linked below.