What usually put out the most heat the driver or lead?
The LED.
Neither. My wife when I'm the driver. The heats from her anger.
What scaru said but thats not to say that the driver does not get hot. The more current the more heat.
The led by far. At its worst a 7135 driver dropping 1V from 4.2(which sags a bit anyway) for 3A is 3W. An XML at that current uses ~9W in a much smaller package.
I wonder what portion of those 9 watts is actually used to produce visible light?
25-50%,it depends which led you’re using, and at what current.