Originally Posted by Imwetoddid
Because that totally helps Miss Fortune.
They aren't the same ability though.
Cooldown: 15 seconds (9 seconds with max CDR)
Duration: 2 seconds for AoE, slow lingers for 1 second.
Cooldown: 10 seconds (at max rank) (6 seconds with max CDR)
Duration: 4 seconds
Not only is Varus slow more powerful (and is going to do a ton more damage since it scales with AD and deals damage upfront), but it has a longer range and has a much better total uptime.
Varus: 66-67% (however you want to round it).
So in general, Varus ability to slow > MFs ability to slow by a longshot.
Has better damage
Slows for more
Has a much better uptime
Has a longer range
Has a healing debuff