Well, when you move the mouse to a clickable location, let it there, and start typing, what all happens on the moment that 'focus' changed?
As soon as you start typing, it (dunno what 'it' here is - windows itself?) makes the mouse cursor disappear, until you move it again.
So which other things may happen at that very moment of mouse cursor disappearance? Since mIRC shows a CPU usage peak, it does alot work there, what can that all be?