Since you’re a new user, I recommend reading the FAQ first. It anwers a lot of questions you’re bound to have.
Here’s the part where they talk about waiting for reviews:
I just finished my lessons. Why do I have to wait?
WaniKani has a very specific order. Each item builds upon a previous item, which means in order to unlock new things you must know everything before it. Because of this, the beginning can seem pretty slow. You’re building up a foundation.
In order for the SRS to be effective, you have to wait between reviews. Our goal is to make you review something right before you forget it as this is builds a stronger memory. The first set of radicals will probably take you 2-3 days (or less) to finish if you do your reviews on time. After this, you’ll unlock kanji. Then, the kanji should take around 2-3 days to complete. This adds up to a little less than a week of study time to complete the first level.
By the time you reach Level 2 or Level 3, the speed will begin to pick up. This slow start is making sure that you’re ready for the later, more difficult material. Think of it like a filter. If you don’t have the patience to get through the slower starting levels, maybe you don’t have the patience to learn all the kanji. Language learning is very long-term, after all.