Why does it take so long for my iPhone 4 camera to take pictures?

After I press the button, at least 3 seconds pass before the shutter clicks.

Maybe you have to close all the apps, it will help. Or it's because you have too much thing in the iphone like music pictures etc. How older it gets, how slower it will be

Your problem is probably what's called shutter lag, which is defined as the time between the shutter being pressed and the photo being taken. If you're trying to take pictures of things which don't have an obvious subject for the camera to focus on, it will spend time looking around the scene, trying to find the most obvious thing to focus on before it shoots. This is probably what's causing the delay.

Find a scene with a clear subject and aim the camera at it. It should shoot a bit quicker. But given that you're using an iPhone, it will rarely shoot as fast as you want it to. The cameras in those things are basically for convenience, not quality.