Too self-critical – although, admittedly, this is also something that a lot of people, myself included, still struggle with! So, technically both! 
How I dealt with burnout? Slowing right, right down. More specifically, adjusting my overall pace so that it was both a) sustainable over a long period of time (i.e. slower than I had been going), and b) comfortable enough that I could regain my sense of enjoyment and fun and interest in studying Japanese for the purposes of learning Japanese – rather than the somewhat artificial goal of advancing through the WaniKani levels as quickly/‘efficiently’ as possible.
So, my approach is now: Is it too much, too fast, overwhelming? Slow down. Is it too hard, too many mistakes, frustrating? Slow down.
Indeed, there is a forum post/group for folks who want to try this approach, called 
Let’s Durtle the Scenic Route 
. If you’re feeling a sense of burnout already, you may want to check it out.
TL;DR: I’d say it’s definitely not a ‘sprint’, but for me it’s not even a ‘marathon’; instead, it’s a ‘pleasant stroll’ in a lovely park.
By the way, on wkstats, don’t focus on your average. Instead focus on the “Typical Level-up”, aka the “Median” time on a level. It is much more stable and less prone to ‘outliers’ throwing it off, such as if you have a bad or busy week, or if you go on a vacation forgetting to turn on Vacation Mode.
Think of the median as simply: One half of the numbers are below the median, and one half are above the median. It’s the ‘half-way’ point if you were to list all of the numbers in a list and pick the one right in the middle position. That way, if you end up taking a 1 year burnout-recovery break (for example), that will just tack on a 365 at the end of your list of numbers, but the middle number will still be representative of your overall/typical time on a level,