On Agnarr my bard has 141% haste inherently, with a 41% haste item.
Casting this on self, she dropped to 45%, which means, as others have stated, this spell removes any haste effects on a mob and THEN reduces their attack speed by 55%, Not too bad.
Notice 12s recast timer so not ideal to slow a pack of mobs in camp.
In which case use aoe Melody of Mischief which reduces all mobs attack speed by 43% and, being an aoe song, has no recast timer.