The Spirit Realm

Ok, I keep seeing this over and over. I assume that Partially sucessfull is better than Slightly, but what are the %'s between the two?

Slightly is about half the effectiveness of Partially. What proportion of a full slow Partially represents actually varies by zone, however (higher tier content resists slows better), so exact effectiveness has to be tested empirically.

Testing is typically done by logging the melee of a warrior-type mob in a relatively safe situation to get timestamps on how long it takes to get some number of Kick attempts slowed & unslowed. Trying to count melee rounds is a poor idea because double attack & flurry can mess those up; trying to time the gap between single Kick attempts introduces a large degree of human error plus you're likely to encounter a "fatal" lack of granularity in the game log at some point.

Anyhow, once you've calculated the Partially value for a mob, you know the base for the whole zone, and typically all the other zones considered the same tier of content. Anything that's Partially in there will be slowed the same amount.