I don't know if that's the only discernible difference.

While 3 minutes is indeed an arbitrary limit, the difference between short and long form videos is very noticeable. Long form requires another form of attention, focusing more, more commitment, less distraction; there's even a form of "delayed gratification" (a form of attention that only grownups can provide) in that the payoff isn't always immediate and can sometimes be very delayed.

Short form is like junk food, zero friction, instantly addictive and doesn't require you to really pay attention. Surely the immediacy of attention it needs is completely different to long form video.

I also disagree with your other comment that maybe long form can promote similar consumption habits (you call it "overconsumption"); I don't think anyone can get "addicted" to long form video, it's simply too time-demanding, you don't get a "fix" and the "zapping" effect of quickly moving from one video to the next.

I probably spend 1-2 hours per day watching content on youtube (and much of that is at ~1.5-1.75X speed)

I don't know what qualifies as "addiction", but it is typically where I get my news, where a web-series I watch is released, and where I learn about social justice issues important to me, through video essays.

I'm sure my consumption is very different from that of someone who watches 100 1-minute Tiktok videos per day, but I think it's worth at least questioning how this might also contribute to cognitive performance and mental health.

Though I think a big difference with short-form content is the autoplay functionality (as your sibling commenter mentions). I watch videos which are released by channels I subscribe to, and occasionally (maybe once a week) watch something Youtube recommends to me. So I retain some agency over my viewing habits compared to someone whose decisions are dictated by the algorithm, which also has an incentive to keep people watching as long as possible.