Backgrounds, 100%. Especially when it's a really busy background like a classroom or a library, haha.
On that note, I agree with the perspective thing, drawing backgrounds in perspective is awful
hmm... probably colour? i love adding the finishes and figuring out new palettes, but once i have a palette sorted, its just outline and fill, outline and fill, and in that time im seeing my pages again and again and i start to worry that theyre boring or confusing. snot fun.
Personally, I'm just impatient with how inking takes SO long.
But the thing that's most tedious? Getting better. I just...Kind of put things off and I'm trying to stop that.
Absolutely line art. Thumbnails are fun, sketching is usually fun, inking is fun (different from lineart in my process), colors are relaxing... but lineart is just a pain. It's especially grating since my cats stole my wacom pen and I can't find it, and I'm stuck using this little lenovo pen, which started bugging out a while back so I get jerky lines more often than not. This means I have to be even more methodical and slow in order to get decent results. Also, lots of ctrl+z.
I think this answer changes for me every so often. I remember finding flats really tedious just year or so ago. Now I enjoy that part.
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..!
