Zoro is fast but his fighting style is not as reliant on speed. Sanji has to dodge/evade, he has to cover distances between himself and his enemy since he posses no long to mid ranged attack, his offensive prowess relies on volume/combination attacks meaning he has to land more hits in a shorter time frame, since he is using his body as a weapon-to produce more force he has to strike faster (speed=weight) and because his fighting style is all focused on his legs, the techniques he develops like skywalk/blue walk/pseudo-invisibility are all focused on speed and lending to his fighting style which is centered around that attribute.
So it's not that Zoro is slow, it's just that his combat style doesn't rely as much on speed as Sanjis does. Which is why Oda will never highlight this attribute for Zoro as much as he will for Sanji. But Sanjis offensive ability will never be highlighted like Zoros, since just like speed for Sanji, offense and everything that falls under that tree is more integral to Zoros style.