Note: Some of these I think are bad writing, but others are just personal preference. I'll stop reading with:

-Just narration boxes telling me about the history, world building, backstory for over a page.
-Gets over three main characters without introducing a girl.
-Introduces three dudes and a Romantic Interest, especially if she is Fiery or Super Sweet
-Main character is the standard Relatable Person with no particular heroic qualities who is handed heroism.
-There's a mean spirited joke at the expense of an "ugly" character/all the good characters are pretty and a villain/bully/jerk has their unpleasantness shown by making them fat, acne scarred, or disfigured.
-Cutesy moe girls and A Sea of Super Muscle Men.
-Romantic interest introduced being genuinely unpleasant or creepy to the lead in a way that's meant to be sexy.
-Under-researched theology, especially over-researched Creepy Satanism by an author who didn't learn the historical context.
-Art is very focused on snot/vomit/sores. I think it can be really cool, but the over the top gross style makes me viscerally uncomfortable.
-I can't tell the characters apart

For length, I don't mind a slow start. What's important is at least one plot hook happens before I run out of updates. I need something to follow for. My comic starts slow, but I hope I managed not to post any pages before I had one with a hook in it.

I don't particularly mind the slower start if things are happening. It's fine to establish a mundane world, but stuff happens regardless. If all the main character does is read for hours, don't show it in real time. Show a montage or a fast time elapse.
If nothing happens, I get board and want to look at something else. But since it's a movie, I'm stuck looking at what was filmed/animated and it pulls me out of the world the people are trying to set up.

I'll parrot @eliseswarren in that I really don't mind slow starts at all, BUT I do mind if it takes forever (in real world time) to get to something interesting. The story/comic should be able to pique my interest in the first sitting. I'm also a big fan of stories that start out with a larger-than-average first update that just gets you through to the interesting part. Like if it takes 10 pages to get to the good stuff, I don't want to sit around for 2 and a half months, page by page, waiting to see if I'll like the story or not. I wanna know now xD

I actually prefered a story with a slow start because it gives more chance for the story to build itself (for the better or worse). I wouldn't even mind if the story doesn' even introducing the MC right away and we're stuck with the world history for a few chapters and all, but if the slow start doesn't accompanied with a characters development, good writing or 'smooth' plot, I wouldn't hesitated to drop the story once and for all and probably cursing the time wasted for reading the stuff.
What I hate the most is if there's too much info dump that's gave away everything that's happened in the story in one chapter, as it will destroy the story (in my opinion) because it's too rushed.
But likewise, I also love the fast-paced story if it is presented with a good writing, plot and character development.
So basically, I just read anything which arose my interest and have a good writing, character development, and good enough plot (how many times have I said this?).
Anyway, every person have their own preference.

The prevailing wisdom is to hook the reader early. Whether it be the first 12 pages or first chapter, but I disagree as people are different and what's interesting for one is boring for the other as proven by the replies. All the techniques taught in writing classes are nothing but self-fulfilling pseudoscience. But to answer your questions...

The intro already lost me from the start. I don't care about the intro. I'm just waiting for the plot to start.

If the reason I started reading in the first place (the premise) is interesting enough, then as long as it takes. Otherwise I've already skipped to the middle to see if it went anywhere, and if it's nothing as described, then it's dropped. For example the movie Rocky. The plot doesn't start until half way into the movie when he gets the offer to fight the champ. (Half the movie and there's nothing going on yet lots of people like the movie) The first half really doesn't have anything to due with boxing. Instead it's character heavy. Considered dropping because where's the fighting? Saw there was fighting at the end so I waited.

If it wasn't what I thought it was.

There's no different in mediums to me. As I said, I'm willing to wait.

The intro to Re: Afterlife8 is 3 episodes. First episode is setup of the setting. Second and third episode is setup of the goal. But after that, the character does everything but the goal which was the point.

To be honest, I'd say it depends on a variety of factors. The medium, the characters, the dialogue and the story itself.

I have a short attention span and in terms of comics and novels, if I'm not reeled in by the first chapter, I'm already too bored to care. Even if the start is boring, if I look at the description or summary, and it bores me, I'm not even going to try. And there's also the cover. If the content was all it was to the medium, nobody would give a crap about well done posters or covers or ads. The covers and posters, trailers, etc., are the first things your potential reader/watcher is going to see and that is what is going to give them the first impression of your work. If it looks half-assed, they're going to think the content is the same if not worse and this is how my own mind works.

Now say I pick it up anyway even though the blurb left me yawning and the cover made me want to drop it. That first line in your comic or movie can set the mood of the reader like a snap. It needs to hook them, and the last line needs to make them want to read more. An example of this would be a comic on here know as Rock and Riot. It has so many things that appeal to its target market; great art, top notch dialogue, interesting characters, people in the minority group that they could relate with, etc. If I'm not hooked by the first line of the first chapter/episode to the last, and it's slow as heck, I'd probs drop it.

My exception to this would be the characters. Characters are my favorite part of any medium and if the person set them up well, I'd be curious to carry on and learn more, see how they grow and will probs get so attached to them I'd re-read/re-watch it again and again (This was me with the movies Paranorman, Spirited Away and Black Panther).

And uh...sorry about the long reply. I got too into it haha

Slow starts are only bad when misused. You should know why you're slowing down the start. Is it to build characters? Set a tone? Build up to the fast-paced action to come later? I think a slow intro can work in any genre if it's not pointless, just like a fast intro.

As for me, I don't usually leave a story in the intro. I do linger a bit before giving up on something. That being said I don't leave if I feel my time is being wasted. If I know nothing about the characters or setting by the end of Ch1, I feel cheated. And not just text dumps of "lore", which never works. Whether it's slow or fast doesn't particularity matter to me as much as characters.

Treating the reader like an idiot by stating the themes straight out of the gate. I don't mean a tag line like:

"Love and war collide in this steamy WW2 Japanese 'invasion'"

I mean stating "Hello, this story is about love and war, you may find some scenes quite upsetting but don't worry, I'm here for you. Cuddles."

The first chapter of an episodic comic should be a vertical slice, establishing who the characters are and their main threat/every day problem. If you can't establish that then you have yourself a problem.

My comic, Althea Sven4, is still in its introductory "chapter", but I've been making sure to focus on character dynamics and doing as little information dumping as possible simply out of personal preference, but still getting the general format across. World building can come later.

Personally, I like some slow starts. It depends the type of story it is. Especially when it comes to movies and comics, because it's all very visual and I like to be eased in. However, when it comes to a novel, if it drags on for the first entire chapter, you lost me and I won't continue.
For a novel, I tend to stick of Mysteries, horror and true crime books for the simple reason that they have a hook on the first chapter. Usually not very long either, but just something to catch your attention. I'm a visual person and reading long paragraphs bores me to no end.

For a comic, I drop a story because:
- there's no distinction between character.
- have to wait a long time to get a new page up and I don't know what the plot is yet.
- the artwork is too inconsistent or the quality drops over time.
- the characters are too cliché and have no character development.
- A chapter ends with no hook to continue, so I just assume nothing will ever happen.
- The first page is a long novel like backstory. (I get bored and lose interest)
- The backstory is over complicated and long, but told all at once and not brought up until much later and you're even more confused.
- The plot is too simple and obvious. Meaning it's good vs evil and good always wins, but there's nothing in-between and you basically guessed the entire story just by the first chapter.

I personally like slow starts for a series that the plot is more complicated and/or might introduce a lot of characters. Gives a chance for the reader to keep up, you don't want to confuse or overwhelm the reader. If there is a backstory, keep it short and simple and build the story with time or if it's really complicated and it has a lot material, just incorporate it in the comic over-time so the reader can learn it slowly within the story.

My series Megami Shiizun8 is definitely a complicated one with a lot of layers and quite a few characters. It's gone through a lot of editing and re-work for years. It needs a slow start or else it's too confusing. I want the reader to be asking the same questions as the characters on their journey. You can't rush it.
However, when I introduced the series to Tapas, I uploaded 17 pages in one day. It gave the readers enough material to be hooked.
And, the first page is the first hook, I did a sort of synopsis to make the reader turn to the 2nd page and so on. I didn't want anybody to doze off by over explaining things that I would eventually be repeated as the story moves forward.

Personally:

Walls of text. If people get the urge to press a button that says skip intro, then is not a good intro.

When they fail to make me care or be interested in the characters. For me, characterization is vital.

If it feels too generic and if they don't give a twist to the apparent cliché. If i feel like i already saw this story many times, then i may leave, except if somehow they made me emotionally invested in the characters.

If the pacing is too slow. I have patience for a build-up for something happening. But if the build-up leads to nothing or if it feels like empty filler, then i lose interest.

Redundance. If they keep repeating the same over and over, they lost me.

Those are the points i remember for now.

I think to some extent this depends on what you're "selling," so to speak.

Like, Coraline and Alice in Wonderland aren't the same kind of story. In Alice in Wonderland, the story is Wonderland, so spending a lot of time in the real world would be wasting time. But in the Coraline movie, the story is Coraline and her relationship with her family. The world she enters isn't a fantastical world to explore, it's an eerie mirror of her specific life. If you're not willing to invest in Coraline herself before the fantasy elements come in, that's fine, but it probably means you're not actually here for the kind of story that Coraline is.

Your beginning is a promise of the kind of story you'll be telling. I think you shouldn't waste any time making that promise, but what constitutes the promise will be different from story to story. In some stories it will mean getting to the action -- in other stories it will mean getting to the characters.

For me it's grammatical mistakes, especially spelling errors. I just can't deal with them. It hurts my head. But if the episode is interesting enough, I usually stick around and leave some feedback so that creator's can edit it.

I tend to only subscribe to comics with ten or so pages already, since for me one page isn't enough to judge my interest in something (one long update would work in the same way.)

I feel like I don't notice slow intros that much in stories. What bothers me more is slow middles. The intro is enjoyable because I like knowing about the characters and the world, which a good intro should establish. But when a comic spends a lot of time on one scene after we've been introduced to the characters -- that's what bothers me. It's because I lose the thread of the story after sometime. And it doesn't help that I read a lot of comics on this site!

If I pay for a book I'll always read it to the end, unless it causes me some kind of distress or it's poorly written to the point of being offending. No matter how laborious the middle is, I'm a speed reader and I'll finish the book soon enough.

For a comic that updates weekly, if I can't follow the story anymore because of a dragged-out middle, what I usually do is continue to click on updates and press the like button, so the author can earn some ad revenue and feel good about getting likes.