After thumbnailing all my comic panels for an episode, I actually completely finish each panel one at a time. I tried doing everything in an episode step by step (sketching > line art > coloring > shading) because I thought it would be faster but it ended up taking me longer because I got bored quickly!

I feel like if I did one character or certain parts at a time, I'd probably confuse myself. I might try it some day!

I normally do one panel at a time, because i've found putting what i want in each panel makes me go, by the time i get ot it, 'wait no i don't like that' and redoing it, thus adding more time

I try to do mine as fast as possible, panel by panel. Seeing a whole panel's line art complete also motivates me to continue drawing more compared to seeing only parts of it finished.

it depend on the panel
if it's one i'm excited about i'll do it first and then the rest,
or if it's relatively simple, like one of my most recent ones is literally just a few squares

I do one page at a time, one panel at a time. I draw the whole page with pencil. Ink the whole page with pens. Color each panel one at a time with markers. And fix my linework one panel at a time with the pens again. I only skip a panel if it's a big, involved panel and I don't have the time or energy to deal with it properly. Sometimes, instead of skipping it, though, I'll color just part of the elements (like just the background) to get it out of the way, and finish it later.

I tried doing the 'color a specific thing in each panel before moving on to the next thing' (like coloring all of the skin tones before anything else), and I found it... confusing and easy to miss something and have to go back and fix it. Doing one panel at a time helps me make sure each panel gets the attention it needs, and is fully finished and cohesive. 'Batch coloring' results in panels that... look like colored pieces instead of a cohesive whole, for some reason.

I'd like to be speed, if possible

the answer to your poll is whichever one feels most comfortable to you.

The answer to the actual question at hand is to be less precious about your art.

There's two schools of thought when it comes to comics:

1: each page is a finished work of art and should be appreciated as such.
2: Each page is part of a larger whole, and the individual images are meaningless without the rest of them in conjunction.

This is a sliding scale, not a binary, you can be right in the middle or lean towards one side or the other or be firmly in a single camp.

If you lean towards 1, then who fucking cares how fast you are? Make each page beautiful and incredible in its own right. That takes as long as it takes, and everything should be as perfect as you can make it.

If you lean towards 2, then the solution is to be less precious about your art.

Trying to draw a circle and the lines don't quite match up? Who cares, move on.
Doing some cross hatching that isn't perfectly aligned? Who cares, move on.
One eye is slightly lower than the other on a background character that isn't actually a focal point in the scene? Who cares, move on.

I feel physical pain when I watch artists take 5 minutes to draw a single curved line because it doesn't line up 100% perfect with the underlying sketch, so they CTRL+Z, redraw, CTRL+Z, redraw, CTRL+Z, redraw, CTRL+Z, redraw, over and over and over again.
The 'Undo' function is a tool, not a crutch. Do not become dependent on it.
99% of those little mistakes that make you a slow artist DO NOT ACTUALLY MATTER. Nobody, and I mean nobody except for you is actually going to notice them.

Let your sketches be loose and messy; focus exclusively on form, perspective, and composition when penciling. Trust yourself to figure out more details than you think you can in the inking phase. You might surprise yourself.
Line doesn't match up perfectly with the sketch underneath? Who gives a shit? what you need to ask is: 'Does this line look good and communicate what it needs to communicate?'. 99.999% of the time that answer is yes, even if it doesn't line up as well as you think it should.
If you lean towards number 2 and you want to be faster, then you need to understand that your art is not some precious little baby that needs to be polished to perfection, it is a small piece of the pie. A cog in the machine and nothing more, so get it to where it works, and then move on to the next cog so you can get that machine up and running.

Oh, and also, don't zoom in. If it isn't visible at 50% zoom on your screen, it will never, ever be noticeable in a finished/published version, be it print or posted online. If you ever zoom in above 50% zoom, limit yourself to 10 seconds before you zoom back out at most. Do as much of your art as possible from a reasonable view, otherwise you will get lost in the weeds for a bajillion years on pixel-by-pixel perfection.

Speed is literally the main thing I care about at this point in my career until I can afford to slow down.

I ink and letter and everything traditionally and what works for me is going through every panel and doing all of the panel borders, then lettering, then inking the characters, and then the backgrounds.

Fully inking one panel at a time really severely slows me down so every part of my process is very much like a factory lineup. Even with my colors if I have a color selected I go through the whole page and color everything in the selected color that needs to be that color.

I do panel by panel but I deffo don't go in order. Especially if I need to do some background or detail work, I may either bite the bullet and do those panels first, or at the end. Mostly first, so then I have an idea if I'm running out of time and need to edit down other panels (which I fully admit I absolutely do. I'll put in a silly chibi face on a character because I had to do a background 4 panels back. It's fine, no one will notice)

Bear in mind I am talking about scroll format. When I do page format comics, I will tend to start on the most detailed panel and then work backward, but only after I've laid out the entire page with the text.

When I get to inking, I usually ink all my backgrounds first in panel order, then go back and ink all my characters in panel order. It's easier for me to do the parts I don't like as much (backgrounds) first, so I can end with the fun part (characters). I always do things in panel order, rather than hopping around, because it helps my brain track my progress a bit better. I also work in chunks instead of individual pages, so over a batch of about twenty pages I'll go through and pencil all twenty pages, then ink the backgrounds on all of them, then ink the characters on all of them, etc. It helps me maintain consistency across a scene/chapter, and stay in a groove.

In addition to what someone else mentioned about trying not to get caught in an endless Undo/Redraw loop, I also think it's helpful for drawing quickly to not zoom in too terribly far on panels so you don't get caught up inking little details that won't be seen zoomed out anyway. Also, I like to keep my pencils very loose, which saves time at that phase, and allows me to have a little more freedom for wiggle room with my inks. And if I'm doing backgrounds from a 3D model, I don't do pencils, I'll do my final inks right on the 3D model.

I tend to go panel by panel, but not necessarily in order. Sometimes, I'll skip through and line all of the headshots for one character, for instance, because I have the strokes for their eyes and nose fresh in my muscle memory. Then, I'll go back and do the headshots for other characters. I also often line all of the background elements first, to get them out of the way.

Most of the time I just work in order, but skipping around can help me when I'm feeling really low in motivation.

one thing that has helped me in inking is this: after I do my rough sketch, I'll add a second layer of more detailed, yet still messy sketch. As I'm going through and inking, if there's something that's not quite right, I'll switch back to messy sketching for a moment on the lower low opacity layer so that I've got a better idea of the anatomy so that when it's finally time to ink, I can just quickly, ink ink ink, with one swipe per line, to get it done quickly and have steady lines. so the actual inking part goes by quickly, it's preparing to get to that final line that takes more time.

I do the lineart for the ones that are easier to save time and leave the complex ones later. Also because it gives me confidence that I can draw (lol), most of the time it's lineart for one character at a time.

After thumbnails are done, I just work on whatever I feel like - ends up being a different order every week. Since I have the deadline, everything gets done but I tend to prioritize panels that are fun and important and "rush" the rest. Works well enough.

I once tried to finish one step throughout the whole update before moving on to the next. Like all sketches, then linearts, colors, and so on. It helped with consistency and it did feel I worked a bit faster. But I found myself forgot how to sketch the characters when it's time to do the next update because I didn't do sketches for days and focus on the shading step. So I went back to finishing one panel at a time.