The entire shading process,, I have a particular shading style I started using last year, and I wasn't planning to use it for my comic to save time, but I really like how it looks on my characters so I decided to anyways. It is by far what takes up the most time when making my pages, especially if a bunch of different characters are on the same page. If it's mostly one or two, it's much easier.
Though I have a feeling that backgrounds in the future will absolutely suck there's no doubt about that haha