This so much. It's actually a rule many Japanese comic magazine editors have when critiquing a comic since competition is tight as hell. If the first 5 (or 3 in some cases) isn't interesting or attention grabbing, they don't think it's going to be able to get enough votes from readers and make it to the next issue. This is talking about readers who already have the magazine in their possession, so even if they didn't find the premise interesting, the middle of the chapter can catch their attention when they flip through the pages.
But webcomics? There's the "next page" button they have to click and they gotta wait for the page to load. Or they gotta scroll, which some people have already expressed can be tedious. And on platforms like Tapastic, there are ad banners and icons of other comics on the same page. More resistance and distraction. So in many cases, the first few pages of a webcomic have to count because there are definitely readers who aren't patient enough. And unlike a book or a magazine that they can pick up, read again and notice things they haven't before, they can simply close the tab of a webcomic and forget about it forever.
Back to topic, lol:
What i notice first is the quality of the art, like, whether or not it's easy on my eyes. The drawing and anatomy can be well done but if you give me nothing but vivid colours, or colours so pale to the point I don't know where I should be looking, or simple lines for with hardly any depth (gag-a-days don't really count), then my interest might fade quickly (my eyes are already tired from staring at a screen all day at work. Please don't make my eyes work even more). And then I will notice if I can AND want to read the thing. Text that's too small, handwriting that's illegible and exposition are big turn-offs. And the last thing I notice when I see a comic for the first time is whether or not it's welcoming me with room to breathe. Clear shots of the characters, establishing shots, and a gist of the world of the comic. Panels that aren't too small or crammed in the first few pages works great for me. I am unfamiliar with your comic and I'm not invested in it yet, so I would prefer to walk into the lobby first rather than be tossed straight into what feels like a busy office floor.
Only then I will make the effort to properly read it and see if the writing fits my taste lol. (I'm a visual-ass person)