Last year most levels of the difficulty category required some slowdown to play (that is to say, most levels had a section where I decided to use some slowdown, all of them indeed at 1/2 speed). Something requiring some slowdown can indeed still be Kaizo, but you should be very careful with sections requiring slowdown since it can get tedious very quickly (and, as always, you are at risk of players using slowdown everywhere
since it is required somewhere...).
Pit levels are usually a notch above that, requiring slowdown almost everywhere. Some of the older Pit hacks (such as the first ones made by Moltov) can be played at only 1/2 speed (iirc dainaguy TAS'ed them at half speed) but more recent levels definitely require frame advance or heavy slowdown (1/5 or even 1/10 speed or so). Requiring anything over 1/3 speed is a clear indicator that the level might be too hard to be called Kaizo.
As for Pit hacks being one level hacks: most are, but not all. My Pitiless has several levels and I personally think it's a Pit hack. I must admit that Pitiless is in fact the only multi-level Pit hack I know, but it's not a solid rule that Pit hacks can't make use of multiple level-ending gimmicks.
Also as mentioned above: adding some safe spots and giving the player spare time can make a lot of difference.
--------------------Of course you should fight fire with fire, you should fight everything with fire.
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Sadistic Designer - testing Pit without tools.