You may want to check for the power-saving option on your devices - do check if it performs better with/without the setting, and also do ensure other apps are not running in the background at the same time.
We do keep a baseline, low-end device for testing purposes, and we try to optimize the app and keep it playable for as many device types as possible. We apologize that it will be difficult to maintain the same level of optimization for much older devices, as we do seek to push forward with more features and updates for ZK2 which would in turn require more performance power.