Editing, translation, proofreading and whatever else fits into the "final touch up" category.
As well as the exclusivity period for Patreon supporters of course.
If you really want the updates to get done faster, then you're gonna have to figure out how to join the dev team since LopaPhi IS a solo dev. As far as I'm aware, he writes, renders, codes, and makes the characters all by himself. Only things he leaves to others is everything mentioned at the top of this comment.
Sure, he could probably speed things up a little and be fine but this game's yet to disappoint me so I'm like 80% fine waiting for it. Maybe 70% if the next update takes 6 to 8 months to make/release.