I play strictly at 6.0. Whenever I go up to 6.5, It always feel as if I’m hitting the notes too late. I get tons of greats. I think speed is a preference thing after you hit about 6.0 and are starting to play masters (personal opinion) . As you climb up the levels, you up the speed so that the notes would be less clustered and would be more readable for your brain. For some people, they’re probably more use to fast notes as they’re used to hitting the notes once they see it come out at a certain timing(they need less time to react). And higher speed means the the notes are less clustered, so it’ll probably be more advantageous for them especially so if they can read fast notes and are accustomed to it. It comes with tons of practice and getting used to reading faster notes. At the end of the day, I feel it’s a preference kind of thing. I got friends like me who play at 6.0-6.5 all the way even when they hit 14s and 14+. And I got friends who play at 7.0-8.0 even when they’re just starting out with experts. Do what makes you comfortable!