The Speedlite 580EX is a powerful flash, with a guide # of 58m at 105mm focal length. This is being shot at only 56mm focal length, with a target only 5m away at f/5.6, and the difference to the built-in flash is quite visible.

You may wish to look at the 1/8s shots with no flash, at ISO400 and ISO3200 for comparison.