How the hell do people go through levels so fast?

Well if it helps you feel better, vocab reviews don’t matter for leveling up and unlocking new items. Perhaps if you spend more time on the individual vocab items when doing the lesson, it might stick better? I find that reading the example sentences really helps me get vocab items faster.

EDIT: Just had a thought. You might be having trouble with rendaku! There’s a userscript called WaniKani Rendaku Information which adds information about why a word does or doesn’t have rendaku in it. Also looking up rendaku rules to get a general idea might help you out! Hope this helps =)