The newest version of the iPhone OS (iOS 4) causes graphics in SmartGo and SmartGo Pro to be much slower. Replaying games, zooming, and scrolling are all significantly slower. This happens both on the new iPhone 4 as well as the older 3G and 3GS after updating to iOS 4. We’re working on a fix and will submit an updated version soon.
Technical details: SmartGo uses the drawInRect method of UIImage to draw each stone scaled to the current board size. With previous versions of the iPhone OS, the scaled image was cached; with iOS 4, it appears that the scaled image gets recomputed each time.
Update: (July 16, 2010) This is still an issue in iOS 4.0.1. But updates for SmartGo and SmartGo Pro that work around this problem are now available in the App Store.