Zoro is a swordsman who clads haki to his swords. It's not a free power up because he has to learn to control the sword from over using his haki. Get it?
We have a high tier in Sabo learning a fruit he just gain, and is getting the hang of it quickly. Why? Because he's not an experienced kid. As for the sword, first, the user has to be a swordsman, otherwise, what's the point? And second, has to be good enough to master the blade.