you should fix this app just keep crashing and is very slow...
What's your device and iOS running?
Performance depends on such factors, not only on game itself. (but yes, it is a bit heavy coded, but they are working on it).
Anyways, It almost never crashes with me, using iPhone 4 (good processor, good ram) with iOS 6.1.
Lag, a bit when opening GvG or when switching back to it from another app.