no idea what about lvl 5 is special.
maybe some guru’d items, and items you were supposed to guru, but failed, came around again.
that’s not a difficulty spike though, that’s growing pains.
this is a still gentle outlook on the path ahead of you, not yet full of nasty leeches you’d have to tackle heads on to get rid of them.
some people say lvl 14 is bad, too. i wonder why. in my experience, wk has been getting more difficult until somewhere in the high 10s, then stabilized. complexity of kanji won’t get up in general (there’s always odd cases like 響), there’s not a lot more reviews at that point yet (that’s in the 20s, when burns start).
once you get to that phase, it is what it is until you hit the fast levels in the latter half of the 40s, but you’re not forced to do them at max speed. in fact, you can always make it easier by slowing down.
the general rule is that what you pour in is what comes out. doing a level’s content in one session will produce spikes. spreading them out generates a stream. 20 items in per day will lead to 20 items out.
similarly, if you have 10 items each 10 times per day and want to consolidate, let them accumulate and do them in one go once the stack is where you want it.
wanikani really is about time management. you plan ahead 2 weeks (guru 2, after that, review times go up a lot), set review piles up to be certain sizes, and if you want to take a break, you stop new lessons 2 weeks before (else you might as well throw away everything not yet guru2, because you’ll forget it).
if you’re aware of what lies before you and how much you have to give, you can set it all up for your convenience.
@jprspereira wrote a more detailed guide on it, including pacing, good companion scripts and how to handle certain cases.
there’s nothing to be worried about, but you cannot slack off. SRS punishes sloppy behavior.