Martin Heidegger, the 20th century german philosopher, theorized that people relate to tools differently than they relate to other types of objects. He called this the Ready-to-Hand principle.

Ready-to-Hand is a term he invented to describe what happens when people become one with their tools. His insight was that people don't notice their tools while using them. Instead, they are focused on the desired outcome. Imagine a person driving their car on the highway. The driver isn't thinking about his car -- he's thinking about how to get to Mom's house in Decatur. He's not focused on his tool. He's focused on his task.

Since a web application is a type of tool, it adheres to the same principles Heiddeger described for physical tools. When using a web application it should be Ready-to-Hand. It's use should feel as natural as waving your arm. While this sounds simple, the reality is a bit more complex. The psychological connection we have with our tools is very fragile. If something doesn't respond as expected, then the connection breaks.

When this happens, we react like a carpenter with a suddenly broken hammer. Curse words fly, and we lose focus on our task. The tool is no longer a tool, it's an object that has to be fixed. The reason people hate slow applications is because of their lack of responsiveness (AKA lag). Our mind is awaiting a timely response that never comes. We can't become one with our tool.

And while speed is a common cause of frustration, the Ready-at-Hand principle really applies to the entire user experience. An otherwise perfect user experience can be ruined with a single flaw. If a hammer slips out of your hand, or a web application has a confusing navigation scheme, then you lose focus on the task at hand. You suddenly become acutely aware that the tool isn't really a part of you. The result is pure frustration for the user.