The problem with Kalista isn't that she's actually affected by slows, the problem is that while dashing every % of extra attack speed only counts as .66%, making attack speed only slightly better than halved on her, and attack speed is really strong right now with Fervor of Battle, and Kalista's Rend also suffers obviously. She also only does 90% AD damage, and her critical strike damage is also reduced by 10%. So your options are either take the halved attack speed, play stationary, or become a master of attacking and moving seperately on her. So her passive is pretty close to useless when you take the attack speed nerf and mix it with her 550 range, and she just naturally has lower damage than everyone else.
So yeah, she's ruined right now, but the slows aren't the issue. Quite frankly, she's actually ruined, Urgot was still easily playable(and still is), and I face rolled with pre-rework Yorick, but Kalista is legitimately ruined as an ADC.