Microsoft have finally gotten to the bottom of why the last 1% of any software installation takes as long as the other 99%. It was due to a 'feature' in the original Windows. In the early days, no software could be updated without a senior scientist being there to sign it off. So a delay was introduced giving time for the junior programmer to go to the coffee room and fetch one. Somehow this remained buried within countless versions of the operating system. Since then the '99% delay' has been copied by software companies the world over, and faithfully incorporated into nearly every kind of installation.
Unfortunately, though we now know the delay is in there, the code is pretty hard to find because none of it is documented. We may have to live with the 99% delay for a few more....................decades.