The slow time shout lets you slow time down to make better actions in a fast pace motion. if you unlock the highest level, time slows down by 1.7x and you move slightly faster.
This. I find it really handy as I play like a battlemage, so I throw spells until I run out of mana, then use the slow time shout so I can strafe any ranged incoming attacks while closing distance and switching to my two handed great sword. You'll really notice how much it helps doing this since switching from magic to a 2H weapons normally takes long enough that any ranged enemy will get a couple shots off while you switch, but with slowed time you can easily dodge all that and take them out before the shout is over, generally. It sounds like you only have the first word so far too - once you get the other two it's pretty damn useful, especially if you want your damage out put to come from a skilled method so you level up more, instead of using a shout for damage.