Good old 99% slows, because slow resistance shouldn't be doing it's job.
Slow resistance will never push a 99% slow low enough to not hit the soft cap of 110. Therefore a 99% slow is basically a 90% slow that Riot wanted to ignore slow resistance with. Why is that? Why have a mechanic if you're just going to keep bypassing it?
if
basically invalidate their main purpose?
brings it down to a 75% slow, which is still high, but a massive difference from 99%. For example, if you're
and get slowed by
, your movement speed will be reduced to 325x0.01x0.5+110, which is about 112 movement speed. With Elixir of Iron or
, your movement speed will be reduced to 325x0.25x0.5+110, so about 151 movement speed, which is a pretty big difference. Even the mastery will give you an extra 15 movement speed, and that's just at the lowest base movement speed you can get, without counting bonus movespeed from items and the like.