everyone except maybe the youngest of children know their origins, and even then spiderman's origin is regularly retold through cartoon series.
we're already vested in those 2, which is the purpose of an origin...they give you a reason to care about the hero's heroics. We're well past that with spiderman and batman. Their origins should maybe be a FLASHBACK or a quarter of an act at most.
I actually appreciated superman returns for that reason; they did his origin like a sports montage like "hey remember how superman got here? great. now lets move on"
nah i'm okay with a spidey origin story because i think it's important to show the audience what peter was like before he was bit by the spider and how becoming spiderman changes him as a person.
batman went through much more anguish and agony on his quest to becoming batman. it tears him down as a character and shows us a side of him we don't want - or need - to see. seeing bruce wayne mope around for an hour? zzzzzz....zzzzzzz
just start the story at batman, it's the best way. we see he carries anguish in his heart, we see he's angry. that's all we need. "oh his parents were killed as a boy... makes sense". but there's just nothing mystifying about the character when they show you every insignificant detail... like how he ordered his bat-masks in bulk from some offshore corporation.
same with superman. i prefer to start the story where is already superman. i wanna see what the world of tomorrow is really like. communicate the origin some other way without spending 1/3 of the movie on it.
any time I need an assist, you always come through homie