Why are iPhone apps so slow?

Question

Something I've noticed about every iPhone app I've tried is that they all have places where they seem very slow and unresponsive. It's true of the games, the free apps, the pricy, popular, "professional" apps, and even a couple of Apple's built-in apps. They all seem to have places where they take many seconds or even a minute to respond to screen touches; bog down and show a spinning beachball for seconds at a time; "queue up" input so that a button press appears to ignored only to actually do something ten seconds later like a poorly made DVR; and lock up for so long that the OS watchdog just kills them.

Because these perf issues are so widespread it seems to me that there must be some common performance pitfalls some system gotchas that are coming up over and over again for lots of different people. I'm not an iPhone developer myself, so I'm canvassing the community's opinions:

What are the most common performance mistakes on the iPhone?

Or, what human factors of iPhone development make it too easy to ship with poor performance?

Answer 1

I think that the performance issues are a matter of perception. Apple has employed animation throughout every aspect of the iPhone's interface, which produces the impression of a smooth, responsive device. The slowdowns you refer to appear much worse than they might be because they stand out from the otherwise fluid interface. If you compare the total execution times of these tasks to similar applications on other mobile devices, I'd guess that the iPhone implementations would still come out near the top.

There's always room for improvement, though, and I'd expect that many of the tricks people have learned in the last year will lead to faster, more responsive applications. Even the development tools themselves are advancing, and that should make it easier to diagnose and deal with performance bottleneck. I know I keep learning new tricks every week for squeezing a little more out of the CPU, GPU, or onboard memory.

I'm still surprised by how quickly people have shifted their expectations as to what handheld devices can do. I'm the author of an open source application called Molecules, which does 3-D molecular modeling on the iPhone. A little over ten years ago, these types of renderings were being done on dedicated SGI Irix workstations. A few weeks after the launch of the App Store, I started receiving emails from people complaining that the application was a little jerky when they tried to manipulate molecules with over 20,000 atoms in their structure. In a very short time, people went from treating these devices like phones and music players to viewing them as portable computers.