Runaan's Hurricane SLOWS attack speed (at least on Thresh)

I posted this before a long time ago, but I think it was on the old forums. Runaan's Hurricane, if there is more than 1 target on the screen, actually slows Thresh's attack speed. I don't know if it has the same effect on other champions, but the way it works is as follows:

• Single target in range: Attack speed is fast
• Multiple targets in range (so extra arrows proc): Attack speed is sloooow

It is not just a small difference. It is a huge difference. Like 50% or even double the attack speed.