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I don't have any words for blatant use of max values to try and get more credibility.
Nasus wither is a gradual slow over the duration, 35 to 95% slow at MAX RANK ONLY, for an average of 65% slow ( max rank )
The attack speed slow being halved, it's 17.5 to 47.5 averaging 32.5 % attack speed slow.
Wither has been nerfed numerous times over the years because players always complained about it.
It used to have increased range, even increased under the effects of ult, and the attack speed slow wasn't halved, it was also 35 to 95% attack speed slow. It was so ridiculously stronger in duels than now I can't even...
Zilean is 40 to 99% slow depending on rank but constant slow, not gradually increasing. It lasts 2.5 s (can be rewinded for double duration)
Yet Nasus slow is more 'spammable'. First because Nasus always gets 40/45% cdr, then because the CD is kinda low (11 s max rank)
In any case if you complain about Nasus slow you're a few seasons late. It was infinitely stronger than it is right now. And it is single target and cleansable, which is kinda garbage in teamfights. Go complain about his R Qcd reduction instead