Nothing slows the attack speed of any enemies.
Nothing slows the attack speed of the player or companions.
The Inventory screen shows your attack speed and DPS, which would be impacted if you attack speed goes up or down. Attack speed is currently based on two factors: the listed speed for the weapon, skills that increase attack speed such as Flurry and Rapid Shot.
A potion that increases your attack speed might throw off the game balance. Same with items that would slow down the attack speed of enemies.
I like the idea of Slow attribute for bows. Bows deserve it rather than daggers since sniping scheme for Archers is nerfed, it is understandable because it is obviously an exploit that can ruin balance but enemies shouldn't always chase the archer so a slow attribute for bows can minimize the need for running as fast as possible