Brave Frontier was made to work with more modern devices with at least 1GB RAM/memory. It's advised to update to a better device but you can try the following:
- Tap Home
- Tap Menu
- Tap Settings
- Turn off volume and battle effects
- Restart device
- Turn off every other app that isn't needed
- Play and repeat the process every 30 minutes to 1 hour of gameplay, depending on how quickly the device slows down.
There is still no guarantee of a smooth gameplay experience even if you follow these steps if you are using a device that is below the minimum spec requirements.