Shading. Shading is the most annoying part of the creation process and I hate it. It's so boring and you can TOTALLY tell in certain pages where I lose interest because the shading is so boring to me!
Bleaargh the backgrounds definitely.
I enjoy drawing characters because I actually have the freedom to make them do what they want, what emotion they have, what they're saying..'
But with BACKGROUNDS, eugh.
I have very little experience with drawing/painting backgrounds and that's mainly because I avoid it a lot of times. 
Drawing with perspective is easy enough, but it's just the matter of finding out which angle is the most dynamic, the most interesting... And making sure it's complex enough but not too distracting... sigh!
Practically everything but the sketching part and the story creation.
All the cleanup and adding all the details and colors takes me forever. Oh and I'm really terrible at backgrounds and perspective, so I just avoid them a lot of times, but if I didn't that would definitely be up there as most tedious. I have this grand plan to one day start incorporating backgrounds after I figure out how to draw all my characters with more consistency. Before I started this comic I was never someone who drew the same thing over and over, so that has been taking priority at the moment, and it does get tedious too.
Adding the text and making last minute changes to my buffer episodes. I usually finish episodes early for my buffer but I can change my mind a lot about things and I would sometimes add more panels, change the dialogue, etc until the upload day. Like making it better is good but it gets really tiring and tedious when I'm constantly changing stuff and especially when I decide to change something on the day I'm supposed to upload the episode...
Aside from that I don't find anything tedious in the drawing part and it's mainly because any part I felt was boring I took out of the process lol. Like I hated linearting so I just use the sketchlines and doing flats was boring so I just paint almost everything in one layer.
For me it's the colouring process, sometimes the inking. Worst of all when there's backgrounds to do because I'm very weak at them 8(
I'm surprised nobody's mentioned this yet because it's such a pain for me but speech bubbles. I alwats mis-calculate how much space I'll need on at least one panel.
When I was deciding the art style for the comic I purposefully decided to make the backgrounds lineart looser, sketchier, so I wouldn't lose my mind over them 
It's a life saver! On the other hand though, scenes with many many characters in them get really tedious for me... It takes forever and at least one in the bunch ends up looking like a moron, usually!
For me it's the part where you pour your heart and soul, all your spare hours, and decades of painfully acquired talent into something for a few hearts and some comments, then you log onto Facebook to see a looping video of hamsters jumping between rugs with one and a half million views.
For me it's the character gesture and expression, it's hard to make them look right and natural..
For background you guys can try Google SketchUp, I see a lot of webtoons use 3d to make their background.
STORYBOARDING. easily. Planning out appealing compositions and making everything seem appealing is tough
Thumbnailing is a headache if I don't have a clear vision of how certain events progress. When I have a clear idea? Get through 10 thumbnails worth of pages. When I don't? it takes me literal weeks to try to figure something out. Then trying not to repeat the same layout. Not repeat the same angle. make sure it makes sense........
Currently for me, it's the whole layouts/sketching/lineart phase. I LOVE inking; I dont even have an issue with coloring- but the drawing phase of making a page has been really killing me short of lately. I've been dragging it out until I get to the point where I say "f**k it" and just do the page; afterwards, the cycle starts all over again...
THIS..!
For me when I making Sang Kong1, due of color blindness the most tedious part for me are coloring or to be exact picking color, I must use ref and eyedrop tool to "steal" some colors.
second part, probably more personal problem when my family see me drawing comic, when I stop and rest they ask why I don't draw and act supportive, when I draw they talk to my back (literally) most of them mockery, when I talk back they use words like "it means we concerns about you". I choose not to listen to them anymore except if it was important, but still.. it's annoying.
Sorry about my English, it's not my native language
For me, it's thumbnailing, or paneling. Deciding which size one panel should be or which form it should have, particulary.
I've always had the hardest time with backgrounds. There are some instances where it's a little less challenging but I tend to avoid then just so I don't have to stress about it.
I use to have a big issue when it came to colors but I think with time and practice it's gotten a little easier. What's harder is making sure that characters are staying on palette and then working on color temps for the setting.
Beyond that I can agree with the challenges that come with paneling and speech bubbles. Spacing things out properly between fonts and avoiding tangents and other things can be really tedious so it's probably my biggest issue 
Drawing little tiny people, like in wide background or distant shots.
Their faces/heads always end up disproportionate to the rest of their bodies and I have to use the transform tool a billion times.
I actually quite like those guys. Although, yeah, I do admit it can induce some anxiety trying to make them different. Isn't there a quote by Renoir or someone like that about how you can only draw so many leaves before you start drawing the same ones over and over?
Maybe it would make a funny story to have a comic where all the characters are made up of those guys, with their busted body proportions and sometimes yes, sometimes no faces.