I can't really say I'd be an authority on the subject, considering that I only just started playing osu! frequently during the past few weeks, but I've noticed that I've gone from mid-tier hard songs being passed consistently to low-mid-tier insane songs being passed on 2-3 attempts. I agree in particular with Arnold0 and Lily-Kun. When I started transitioning from Hard to Insane, I noticed there is a big jump in skill required. I started playing songs which I felt confident with on Hard difficulty, and eventually progressed into Insane. At first, I couldn't get very far, so after a few hours of failing insane maps, I went back to Hard, and they seemed very slow in comparison. It was sort of a rinse-wash-repeat after that, where I started finally finishing Insane maps, and I find myself now completing some Insane maps, even on the first try. I've yet to FC an Insane map, but I'm sure that'll come with time.