5-10? He received a 20% increase slow upon non-isolated targets, and a 10% against isolated targets. 40-60% and 80-90% respectively.
So let's test this using Twitch, a character with a base movement speed of 330.
- This test is Twitch at lvl 1 with no items or speed modifiers.
Un-Isolated: A slow by .4 would be 198 move speed, a slow by .6 would be 132 move speed. That's a difference of 66 movement speed.
Isolated: A slow by .8 would be 66 move speed, a slow by .9 would be 33 move speed. The difference here doesn't matter because move speed modifiers bottom out at 110.
- This test is Twitch at lvl 18 with a full standard build that gives me 419 movement speed.
Un-Isolated: A slow by .4 would be 251.4 move speed, a slow by .6 would be 167.6 move speed. That's a difference of 83.8 movement speed.
Isolated: A slow by .4 would be 90.28 move speed. Obviously .9 would be even slower and for this situation it's pointless to calculate that as it will bottom out at 110 again.
So yeah, disregarding that the isolated slow buff made almost no difference, the un-isolated slow buff made a hell of a difference. In team fights, especially since he can hit multiple targets with his evolved slow, he can reduce at the least half a team to a third of their movement speed. Character speed is very important this season, whether it's dodge somebody's all in burst, or to catch up and finish off stranglers after a fight. Not to mention it makes it even harder for players to escape him when he's ahead. So yeah, it's a lot more than a 10 move speed slow. If anything it's on par or stronger than the infamous Nasus slow that people have complained about for years. I think that should put it into perspective.
P.S. I know the isolated slow buff still makes him stronger as targets with a higher move speed can still be effected by the buff. However, it requires a move speed of 550 for the .8 move speed slow to hit 110. So for the buff to have any effect, it will need to be used against targets with 550+ move speed which is rather rare and not even worth looking into.