They are actually 10-15 seconds. Brayflox's boss intro is about 13s, iirc. (00:23 - 00:36 in this video
https://www.youtube.com/watch?v=CQqjd2Rly3w )
Personally, I honestly don't care if people pull during the dungeons with short cutscenes like that. At best, the cutscene watcher is missing a few autos and maybe an aoe marker as you said.
But three dungeons in particular have very long boss cutscenes, Stone Vigil, Thousand Maws, and Aetherochemical IIRC. All three have cutscenes that actually give MSQ exposition, and all three are about 2.5-3mins long, which is enough time to actually make the cutscene watcher miss out on the entire fight, if not the majority of it. In those instances, people should 100% chill out and go grab a snack/drink or check their phone while the cutscene watcher enjoys their MSQ. Lest you know, enough complaints roll in about those dungeons that Square feels the need to give any dungeon that's not super quick cutscenes (or worse,
every dungeon) the Praetorium treatment and you'll be
forced to waste that 2-3 mins every time, even if no one's new.