Does blizzard actually slow enemies down and by how much?
All blizzards slow enemies by 30%. Grasping Chill has an additional slowing effect after the blizzard is over, by leaving ice on the ground.
Source: My own testing, since none of the books or tooltips suggest Blizzard slows enemies. You will always see "snared" come up on monsters as they are hit by the blizzard though.