GUI.button response time?

Is there any GUI.button response time setting in Unity? The reason is the GUI.button used in my iPhone game seems to have a small response time. Meaning when you tap the button quickly, it will not trigger the "Clicked" function instead it will be "hover over". You need to hold your finger for a little bit longer to "click" it.

Is there a way to overcome this, so on iPhone will trigger "click" once you tap on it? Thanks!