If i understood it correctly... Initializing things as USB, which are inherently plug-n-play peripherals do need to wait for some protocol-defined time before we can say all peripherals on that bus were initialized. (Eg. i beleive USB devices have 0.5 second to enumerate, so if you wait for USB before you continue booting, it's automaticaly 0.5 second delay. But if you find USB hub, which is rather common, it's another 0.5 second delay to enumerate devices behind it.).