Inking, flats and shading are all hugely time consuming, though I think inking is probably the most so. My sketches are usually kind of bad so I have to spend time adjusting placement of lines and especially faces until they're closer to what I want. (Which I suppose is all right lol, as it helps give me more satisfaction in moving from sketches to inking; I've found when my sketches turn out great it makes inking more intimidating/frustrating.)
In terms of each individual page, storyboarding goes relatively fast (I call it storyboarding as opposed to thumbnailing as I prefer to plan and place my sketches directly into a document already regular page-sized), but it's by far the most mentally demanding part of the process. Admittedly I used to somewhat devalue the storyboarding step and feel like I should be getting it done faster, and try to cram it in during those low energy times I was tired and not up for much, but now I give it prime importance and focus. Framing is such an important part of achieving drama and emotional impact, now I'm willing to put more time and energy into it if I need to. Possibly storyboarding feels the longest of all steps even if it's shorter, because of how much intense focus it takes.