At first I thought line art, since it feels like a slog to get through, but I think it's actually coloring that tops it.
At least with lines(for me, that is), there's just one color, one pen, and at most 2 layers (fg and bg).
When I color, there's a ton of layers for each element(ex. 3 layers for hair, 4 layers for irises), I have to color pick for continuity(ex. returning to a past location), decided what to color the people in the bg, and add glows, weather effects etc.
It's especially obvious when working on more than one page at a time. Sometimes I'll think I'm finished when I realized I never colored my lead's shirt buttons.