Some steps for an improved overall performance of your device:
1. Free up your device’s memory by closing applications running in the background.
2. Make sure that your device has enough free storage space, as having an overstuffed device can potentially result in crashes, slowdowns, and other unexpected issues. Enter your Settings -> General --> About and check both "Capacity" and "Available" . Delete any unnecessary apps, videos or other big files that are just using up storage capacity.
3. Check if your device's version of iOS is up to date by entering Settings --> General --> Software Update. Follow the on-screen instructions to download the most recent update. If your iOS is on version 4.3 or lower, you may have to connect your device to your PC or Mac via USB, launch iTunes (on the PC/Mac), and follow the on-screen instructions.
4. Have you tried turning it off and on again? As with many things, restarting your device and reentering the game may help.
5. Make sure that you're playing on a stable internet connection (preferably WiFi, 3G or 4G). You can also try resetting your network settings by switching Airplane mode on.
6. Turn off unused Bluetooth connections (unless, of course, they're required for your internet access).
7. Perform a soft reset of your device by holding down the home and power buttons at the same time (you’ll see the Apple logo flash up on your screen). Next, restart your device and launch Evoker again.