I have one level in it, which (with my high dex) gives me a slow-down of 21% (according to the description).
Here's an image of an orc archer that has got six arrows in the air before the first one lands..

Edit: Just watched an orc high cryomancer cast ice-shards, which was slowed down so much that he stepped in front of it, and was then hit by the spell..
[LOG] Orc high cryomancer casts Ice Shards.
[LOG] Orc high cryomancer hits orc high cryomancer for #1133F3#172.88 cold damage#LAST#.