It’s called Wanikani Override:
Also, since I’m posting anyway, basically 8-10 days is what you get if you do WK intensely and don’t allow yourself to make any mistakes that would lower your speed (as in, you can’t get more than ~3/4 kanji wrong (didn’t do the math)). It’s not what one should do, and I’m not even sure it’s the most efficient thing.
Why some go fast
Some go fast because they have precise goals/deadlines. Some just can’t imagine investing 3+ years on WK. Personally it’s because I’ve always felt like Kanji is what made learning Japanese so hard. Just knowing that it’s not an alien symbol, but it’s that character makes learning Japanese much more enjoyable. And I can’t wait for that “another alien symbol” feeling to disappear. Although yeah, it’s a little bit because of the cake too 
If you want to know how to get faster, it’s something we can’t answer because we don’t really know the technical reason why you are taking 20 days:
If you are doing 20 lessons a day, and still can’t get faster, you are probably getting some kanji wrong too many times. Not much to do about it except correcting the typos with Override.
If you are doing 5-10 lessons every day, than that’s the reason. Just divide the ~9000 kanji+vocab by 10 and you see it’ll take ~900 days. Nothing wrong with it, as long as you are fine with it yourself.
Personal opinion
Also, IMO studying three languages is anything but average. No wonder it’s taking you a little bit longer. Don’t set the bar to high and burn out!
P.S. Someone suggested the reorder script. I’m not sure it’s a good idea if you are doing <10 lessons a day. You risk ending up buried in the vocab from the previous levels. No risk in trying it though, you can judge yourself 