Shouldn't Sanji be able to use Rankyaku? Since the CP9 can kick so such leg strength and swiftness that they create a blade of air. Zoro's attacks are all mostly based on that, swinging his sword so the swing leaves the blade and continues on its own, from that point on you just need to improvise to get different variations and that's what Zoro does everytime we see a new attack, of course haki plays a part here and there like Asura.
However, bottom line is that Zoro needs swords for his attacks to travel but Sanji who was stronger then the cp9 can't do the rankyaku for some reason which is very strange. If you can't kick that fast you should be weaker but he is not.
So yeah on topic, Sanji is faster, Zoro is fast but his sword-skills is what does the trick, not necessarily at how fast he himself move.
Though he hasn't shown trouble keeping up, don't assume he's faster then Sanji.