As far as I'm aware, this is different from the Warlock attack speed difference. Whenever Bottom Tree Way of the Sharpshooter's melee skill is up and available, Weighted Knife, Hunter's melee attacks take a half second to a full second before you can melee again, whereas with every other melee option available, up or not, it does not have this bug. Tested in the Whisper Heroic, in the infinite thrallway. To melee 5 thrall, without Weighted Knife up, it takes roughly 4.94 seconds. To melee 5 thrall, with Weighted Knife up, it takes roughly 7.12 seconds. I tested other melees as well, and they all had the same attack speed as the test without Weighted Knife up. This is probably a holdout from Bungie deciding to give you a kind of leeway when fighting enemies so as to not try to waste your longer animation knife when trying to melee trash mobs.