Apple seems to have bloated iOS 8 which makes older devices run slower and less smooth than on iOS 7 which could effect gameplay. We have submitted an update (v6.3) and have added a feature "Optimize for Older Devices". When turned on, most animations during tapping is disabled. The cash icons do not animate and the "+5 XP" banners do not show. Our internal tests shows this allows the app to run much smoother. If you are on a new device, you can enable this setting as well.
We will announce when Apple approves the update, hopefully in 7-8 days.