Many (maybe most) people who level up fast (I consider less than 10 days per level to be a fast pace) have some prior Japanese learning experience.
Without getting into the technicalities of reaching top speed on Wanikani in my experience (shared by at least a few users according to the discussions I’ve had) if you do about 20/25 new lessons everyday, with good accuracy on radical and kanji, you should be around 10 days/level on average.
20/25 per day already way more doable than 40 per day.
That being said people who level up faster also end up spending more time everyday on WK, they are more daily reviews, they spend more time en review etc.
People should focus on finding a good pace that they can follow for a long time but that pace should not be too easy compared to what they can do. Challenging oneself a little bit is not that bad as long as you don’t push yourself too hard.
Edit: Also you do not need to finish all items in a level to level up. As long as you Guru radical and Kanji you can level up.