[chc]

It runs smooth as butter on my three-year-old 3GS. Smoother than a lot of websites, actually. Are you perhaps using a third-party browser or something? Because otherwise, unless there was a serious regression with the iPhone 4, I think something's wrong with your phone.

[geoffb]

It could be due to the retina display. iPhone4 has double the resolution of 3GS. Drawing 960x640 pixels vs 480x320 in canvas is going to have some noticeable impact.

[jmck]

my iPhone4 gets ~60fps in Safari, using iOS 5.1.1 (latest).

[paintAcquaint]

I'm running Safari on it.

[phoboslab]

Are you still on iOS 4? Mobile Safari in iOS 5 had a new, hardware accelerated, canvas implementation.

Otherwise I have no explanation why it runs slow for you. As you can see in the video on the article, the game should run just fine on an iPhone 4S.