If you want to go as fast as you can in WaniKani your first thought would probably be to learn all lessons as soon as they show up. But these lessons will be harder to remember if they are too many.
I noticed that the only factor for leveling up is whether or not all your kanji and radicals for the current level are on the guru. I also noticed that most of the SRS items in WaniKani are vocabs, not kanji nor radicals.
This means that if you want to go as fast as possible, you only need to finish all kanji and radical lessons. This will be easier to remember during reviews because it’s a lot fewer since most of those lessons will be vocabs. You can then spread out your vocab lessons in such a way that it will all be finished by the time your radicals and kanji are on guru. This will make you proceed in the next level kanji and radicals because it was just in time when your vocabs were finished.
It seems like this will be as fast as doing all lessons in one sitting, but also a lot easier during reviews.