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Hi Alessia, try this. Turn off your mobile. Turn it back on after a couple of minutes. Check for updates for your device and apps. If you use Android, you can clear the game's cache. If this doesn't improve the game play on your mobile, then I'd re-install the game. Make sure though that you've synchronized your progress with either your Facebook or your Kingdom account. If you re-install the app, you'll loose saved lives and boosters.
Could you check the free memory available on your mobile device? Lack of memory on a device can cause it too slow down. As you mention that Facebook is slow too on your mobile, this could be the reason. If you use Android, then you could use a SD card to save your game apps and free space on your mobile.