Doing the high levels at full speed is really rough. At the peak, I was spending over two hours a day doing reviews and lessons on average.
Although, some of that of course is that the high level material is just harder in general. I imagine it wouldn’t be quite as bad at the lower levels.
Anyway, I don’t think the Wanikani deliberately made some levels fast or not. It’s just a side effect of the system they created. And some levels have gone from fast to slow and back as they add and remove items.