On numerous occasions, in a variety of locations, I've encountered mobs that after my first hit, go down to 5 or 6%. However, they still take just as long to kill as if they started at 100%. A few examples are the Windwing and Frostwing Talon Drakes in Falgarholm and the trolls in Dark Eyedrop. With these specific mobs it's happened 100% of the time, and I've killed several dozen.