This is an issue that is prevalent on lower end devices, typically with RAM lower than 1GB. We are consistently working on optimising the game for these devices, and will hope to have performance back up to scratch on your device with future iterations of the game. We thank you for your patience on this matter.
For Android users, the issue can sometimes be resolved simply by adjusting the quality in the settings menu. Unfortunately this feature is only available on Android as it's not a feature typically allowed on iOS applications.