I understand how you feel, I felt that way too when I began Wanikani. I even started learning kanji using other programs additionally during the first few levels because I was bored. However, you will notice that you don’t find people above level 10-15 complaining about wait times, because by the time you get to about level 8-9, your reviews start piling up exponentially. I noticed it at level 8, all of the sudden things were getting a lot more intense. I’m only level 13 and I had 347 reviews the other day. I didn’t let them pile up, that’s just how many I had. I haven’t even started burning anything yet, which will increase my number even more. If you can stick it out for the first few levels, you’ll find that things will pick up rapidly. Many people have actually quit because things sped up too fast and they got overwhelmed trying to keep up their original pace.
That doesn’t mean the first few levels don’t feel really slow, because they are, but there is a reason for it. If you’re getting frustrated, I recommend spending this time learning grammar. There are a ton of grammar resources you can check out, both free and paid, and they will definitely fill up your time. You can also start practicing listening. Of course, you won’t be able to understand much at this time, but it definitely helps just to get a hang of how the language sounds.