CodingHorror points out that the game of “find the bottleneck” that is computer performance optimization is always looking for a bottleneck in CPU, disk, network, or memory.

But there’s a fifth bottleneck — a fifth resource most applications wait on. The user.

If an interface is too difficult to understand, or if an action takes too many clicks or keystrokes, the application will be stuck waiting on the user. If an interface is really bad, the application will sit idle while the user is searching for “how to do X in ProApp 8.0,” or reading the manual, or asking their friends for help, instead of working. And the ultimate interface failure, when a user decides to stop using an application, means, from the point of view of performance, that it will never complete — it’s blocked forever.

Sure, a bad interface won’t slow down a computer. But it does slow the user down. And that’s why programmers care about performance – because we humans want to complete our tasks faster, not because we want computers to complete their tasks faster.