Why does Apple's GLGravity example become slow and unresponsive after touching the screen?

Question

To reproduce the problem:

1. Download Apple's sample project GLGravity:

http://developer.apple.com/iphone/library/samplecode/GLGravity/index.html

1. "Build and Go" on a device.

2. Observe how smooth it is. Then, touch anywhere on the screen. Observe how jerky and slow it becomes. Eventually, it becomes unresponsive.

Why? And how would you fix this?