The actual drawing/layout part. I dont do thumbnails so I'm pretty much trying to create the art on the panels themselves; sometimes the art comes together and sometimes it can take a while.
The inking/line art process may take time, but I enjoy it more so than the drawing part so it doesn't seem like it takes long to me. The coloring can be tedious pending on how detailed it is, but I generally enjoy it as well- especially if it starts to come out how I envisioned the page looking. I like doing the lettering as much as the inks, so that process doesn't seem like it takes as long either- unless I run into issues trying to decide a color on the sound FX.
yea even tho line art takes a really long time its at least my favourite part of drawing, but it isn't so fun when you see what time it is lol
Shading characters. For some reason, I can do 6 panels featuring 6 different backgrounds complete with shading and all sorts of tiny details in 2-3 hours... but the moment I have to do some shading on a character my brain goes into "404 error" mode and suddenly I don't know how to draw anymore 
yea, people struggling with the shading process is nothing new, but its something the more you do it the more natural it feels
Definitely lineart, and I'm inclined to say rendering but rendering is the fun part 
But while I enjoy making slick lines, I noticed it really does take up most of my art time truly...
The most time-consuming for me is drawing crowds, cities, war battle scenes, especially an army with their armor. Anything that is very detailed takes way too long.
Inking and toning are the most time consuming parts by far. The storyboarding phase takes me the least, except for some cases in which the drawings are a bit more complicated.
So yeah, it depends on how many panels are there and how complex the drawings are, but regardless, lineart is the most time consuming portion of my work.
Thing that takes the most time is definitely lineart. However, part that's longer than it should, is thumbnailing 
With lineart, it's the longest in the process but still realisticaly as quick as it can be. With thumbnails, I know what is supposed to happen on the page, I know what characters are supposed to say, but somehow planning it out on the page takes way more staring at a blank page than it should.
And text bubbles. I hate text bubbles.
My coloring probably takes the longest. Some things like effects or landscapes I don't ink at all, so the burden falls on the painting portion of my process. And I know that flatting isn't actually the most lengthy part of my pipeline but damn it sure does feel like it sometimes.
Ha, I eliminated lineart from my pipeline with extreme prejudice 
Still, my drawing process is basically 'work on thing X (e.g. sketching panel 4) until you get stuck, then pick different thing Y (e.g. colouring panel 2) to do instead' so I rarely get all of one 'thing' done in one go. Hence it's kind of hard to measure what takes longer
But if I had to give an answer, I'd probably say 'the beginning'. It's just a slog for me until everything except colouring is done, and then the rest just flies by in comparison XD
Coloring. I actually gauge how much time a portion of the process takes by how much of my 'watch later' list on YouTube I watch while I'm working.
Laying out panels and word bubbles? Hour and a half.
Actually drawing the panels? Depends a bit, but usually 2-3 hours.
Inking? Hour and a half.
Coloring? Like... 4-6 hours.
And then I do things like re-inking lines that got faded during the coloring process, scanning, editing, etc... but coloring is the worst chunk by far. Even for easy pages.
For Engram, it's either sketching/visual design (I make up my settings and characters as I go...) or coloring (as in, picking the final colors and shading/adding effects). Easily 8 hours of a 25-ish hours/page journey.
For my other comic that takes me A LOT less time, it's the storyboards. I letter, storyboard + pick colors all together for like 1.5-2 hrs and then everything afterwards takes 3-4-ish hours per page.
Hmmm...definitely the sketches cause that's when I bring my thumbnails to life...unless I completely change the paneling then it's just a little painful cause I have to make it flow. When it's casual (ok not really) sketching and I want to color it, sometimes it can take days, though I prefer just using flats for coloring because it's slightly less time consuming.
For me, it's definitely shading and then in second place, either figuring out new poses I haven't drawn before or drawing detailed backgrounds. Before I got clip studio and a laptop with a touch screen/pen, I had to hand-draw everything, scan it, and then erase messy bits and re-draw lines that didn't scan well with the track pad, so that was a real pain 
At the moment for my comic, probably sketching. Sometimes I'm not happy with how things turn out and I end up having to redo the sketch from scratch, multiple times. I'm not very fussy about lineart or colouring (for the comic), I guess.
As for more detailed illustrations, definitely colouring, by far.
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.