While a server application in general has to be scalable before everything else, a client application is different. It is usually intended to run for one user on one computer, and thus the user expects it to run fast and not cause troubles for the other applications running in the system. While the second part is usually handled by the operating system, the application's performance in getting things done and reacting to user input is crucial for a positive user experience. Imagine if you run an application and it hangs for a few minutes after you click on a button. A good application remains responsive and indicates that you have just started a long-running operation that is still running and is going to complete soon. Meanwhile, you can do something else—click on other buttons and perform some other tasks. When the task is completed, you can get back to it and see the result.

However, achieving just this is often not enough. If you use some application...