Coloring. My webcomics are mostly in B&W since making them full color is very time consuming. I have a work-in-progress comic that I am trying to make in full color, and it is taking a long time to finish the first episode.
Second hardest is doing clean line art. My line art tends to look like rough sketches and I have to do a lot of erasing.
The promoting part is okay since I like making videos of the drawing process and posting them to social media, but I am not sure how many new readers are gained from these efforts.
Mirroring the comic to multiple sites is also a bit of a hassle, especially since the comic has very few readers on the other sites.