Why yao need to be faster than sw?

So I got yao e1s1 at 201 spd and sw e2s1 at 205 spd.
At the start sw could use her skill to get some punchline then get to yao ult to activate sw ult.
So I dont understand why we need yao to be faster than sw. Isn’t it just better that sw could accumulate some punchline in advance?