But, do you slow down too? If you do than it is only a slight advantage. You may be able to see an attack or aim your spell a little quicker but in the end it wouldn't be too useful. Plus they can still tweak the duration at any time before or after release (via patches).
well the bit I left out in the ..... of my quote says
allowing you to casually plant arrows in the skulls of any surrounding enemies
which seems to mean you are still at normal speed, although i wouldn't say that is conclusive.