That's the whole connect at getting at, took a week to learn 2 weeks to master Adv CoA, took 30 minutes to master/near master Adv CoC and 10 seconds to learn it the discussion is lost once you disconnect those two very distinct points though
I would need Sanji to have the need and master future sight to make some speculation thread reasonable using the monster trio and their specialties to present my headcanon but I'll use just use Luffy and Zoro for now to show how their haki skill tree might be working for them
-We have Zoro who specializes in CoA and can seemingly send slashes with haki and injure Kaido with the same ease as flowing haki users like the scabbards so Zoro may find it easy to use and master it.
-Beyond doubt CoC would be his weakest point and he manages to use it in a desperation all out move, since it's new to him entirely it might take longer to grasp properly.
-Now Luffy didn't have the ability to wield armament like Zoro so he focused on that in Udon.
-CoC was his specialty so at the time he gains the ability to wield it he finds it easier than Zoro.
All of this crashes if we don't consider Zoro uses flowing haki and his dialogue about Luffy learning a new haki seemingly confirms that but in case that's just a difference in understanding between a brawler and a swordsman we can make it work, specially if we consider the main mantra Luffy uses to grasp Ryou is the one Zoro has always known.
If we just had Sanji needing and quickly grasping future sight as opposed to Luffy's efforts we could start tying the speed of mastery with specialties and it would play well with their differences but until then I got nothing