There’s a certain accuracy percentage that you should aim to exceed with your reviews, but I can’t remember exactly what it was. Something like 66% or 70% accuracy. If you get below this point regularly, you won’t clear out the items faster than they’re coming back, and it will be hard to progress. If your accuracy is higher than this, you should still be making progress, though you’ll have to put a lot more time and effort into WK than someone with higher accuracy, because you’ll have to do a lot more reviews overall in order to burn the items.
The big trick to increasing your accuracy, I think, is to spend more time on your lessons, and to hit the first two review stages after that very close to their SRS intervals. Some people spend a lot of time looking at the mnemonics when they learn the items and make sure they’re pretty well cemented in their memory (and review their mnemonics when they miss a review). Personally, I drill the new lesson items immediately after learning them by running through the self-study quiz script. After I started doing this, my accuracy increased.
The later stages are a little more flexible than the first few apprentice stages. If you wait a day or two extra on an enlightened stage item, it’s not really going to make much of a difference, but if you wait a day or two extra on a first stage apprentice item (due in four hours), that’s many times the review interval, so you will struggle a lot more to remember that item. Hitting that 4 hour review interval and then that 8 hour one right after will do a lot to increase your overall accuracy on those items.
And yes, exposure to Japanese in native materials or in other forms of study outside WK will also help you remember the items and increase your accuracy, but in my opinion, the two most important things to do if you want to improve your overall accuracy while putting less time into studying are 1) making sure you learn the items well during the initial lesson stage, and 2) hitting the 4 hour review and the 8 hour review intervals as close to those times as possible after doing your lessons.