I don't know about working as in increasing kills, especially since outside of a slayer there isn't a kill goal. I mean, sure there is Conquest and the lower values, but I've never gotten there faster just because I had a slayer boost running.
But there is a bug with slayer boosts and their timer. Occasionally they will count down in quests, which is not supposed to happen. I've been in a quest where several people had slayer boosts running and some had theirs counting down while others did not.