It may help to think less about when the antagonist is introduced, and think about why the antagonist is withheld. I'll use the 1971 film Willy Wonka and the Chocolate Factory as my example. The script was written by the author Roald Dahl so it's faithful to his book. We've also seen a panned remake that focused on the strangeness of the antagonist, and completely missed a the theme about developing a moral compass.
We don't see Wonka until over 40 minutes into the movie. Even though his presence is seeded all around the protagonist, he is metaphorically out-of-reach. The chocolate bars that are labeled with Wonka's name are too expensive for Charlie to buy, while the man himself has been locked away behind his factory gates and hasn't been seen in years. At this point he's not even mysterious, Wonka is part of the world that Charlie can't access because he is so poor, but most things are inaccessible to Charlie, so the symbol of Wonka isn't especially important.
That all changes with the golden tickets, and Wonka begins to build his notoriety with the prolonged world-wide contest which pushes the media into a frenzy. But it's a slow build. Instead of meeting Wonka and learning his self-serving scheme as he stares out the window babbling his plan to minions, we meet Charlie's 4 rivals instead. In contrast to Charlie, these kids are comfortable in ways that Charlie isn't. They each have a problem with excess (their moral weakness). One chews gum, one watches TV, one over-eats, and one is too wealthy. Their "problems" aren't external problems like being poor, in fact the kids are in some ways proud of their problems. Mike brags about how much TV he watches, Violet brags about how long she's been chewing the same piece of gum. They are enthusiastically presented as character quirks, and Charlie seems even more underprivileged because he's never had the opportunity to become interesting. Charlie's never been tempted by excess so he doesn't even realize these are bad traits.
There's a lot more that happens. We spend time with Charlie's humble family and we see them struggle even to buy a single candy bar so he can have a chance at the contest, but we've already watched others hire entire factories with unlimited funds just to open candy wrappers – when there is no golden ticket it's disappointing but not unexpected. The story isn't about Charlie's encounter with some radical magical dude, it's about a sweet kid who is loved but poor – this story is easily erased by the more interesting shenanigans of shinier characters, in fact once we get to the factory we don't spend another moment thinking back about the other grandparents at home in bed. The film sticks with Charlie emotionally, and we have time to actually like him (as opposed to the film just telling us we should like him). We are emotionally invested in him not because he is the cool Marty Macfly badass on a skateboard, or because he suddenly learns he had a Jedi father and is destined for adventure, but because he is a humble kid who we develop honest sympathy for. Rather than being launched into an un-earned adventure where he becomes "Awesome Guy", we see instead how well he handles failure. Rather than wallow in disappointment, he shares his chocolate. Actually Charlie is a pro at handling failure, but this seems to be a story about winning.
Skipping ahead, a false antagonist is introduced as a "spy" who approaches Charlie to smuggle a sample out of the factory – a goal that seems ridiculously underwhelming, it's only one piece of candy and Wonka is just an abstraction. Charlie is naturally repulsed and afraid of the spy, but we recognize he is the same man who approached the other children, and while we didn't hear his earlier propositions none of the other kids seemed afraid. Charlie is already different from the others, but even though we see he probably won't be the one that steals there are 4 other children who will. Even with a golden ticket, Charlie is not the "chosen one" with the sole purpose of saving the galaxy. There is nothing inevitable about Charlie getting the prize. The important thing is that our real antagonist who has been secretly pulling the strings hasn't even registered in the plot yet.
When we finally meet the now legendary Wonka (an earned status because we watch it happen rather than just being told he is legendary), The film continues to stretch out the reveal of his BigBad. The doors open and the cheering crowd falls silent as Wonka limps to the gate pretending to be disabled, only to stumble and somersault and show he's fine and actually a bit of a circus showman. His introduction is so odd, manipulating the crowds sympathy and emotions, but then turns to cheering again as he takes a bow (apparently this entrance was a demand of the actor, so no one would trust the character). We don't have time to stop and be suspicious why he would pretend to be crippled when he's perfectly fine (a false victim). In fact, if we stopped to think "Who is the villain in this film? Who is sinisterly manipulating everyone? Who set a big scheme in motion, and is luring children into a candy-coated deathtrap?" Even the "spy" shows up where the golden tickets are discovered – it's not really a mystery because there is only ever one "strange" character who it could possibly be. The long delay keeps Wonka a cypher, our focus is elsewhere.
One reason the delay works so well is the film is easily divided in 2 parts: before meeting Wonka and after meeting Wonka. It's the best kind of plot twist because nothing actually changes, Dahl hasn't mislead us with red herrings, but everything changes for Charlie emotionally. Wonka shifts from something unobtainable to something you're not sure you really want. The man is still incomprehensible. Is he good or evil? Did those other children die – if not by their own hubris then by the horrible "cures" Wonka promises? Wonka is like the devil in a candy factory. We keep seeing how he lies and manipulates, he lures children to harm themselves and acts bored when they do. There is absolutely no reason for Charlie to sympathize because Wonka is slowly revealed to be a horrible person – everyone actually turns out to be a horrible person. The children fall to their own excesses, their parents make empty threats, even Charlie's beloved grandpa will betray Wonka out of spite. And it's not unjustified.
Charlie makes an internal decision not to steal from Wonka, and it's huge because it's not at all obvious which is the right and wrong thing to do. The film has sent mixed-messages about every character. Charlie is not saving the galaxy by shooting a supervillain who has announced everyone will die when the timer ticks down. There is no angel in a white robe pointing him to a door, he doesn't have a girl to "win". It's not even clear that there is a moral choice to make, no one stops to ask Charlie what he should do. The antagonist/devil sits alone sulking and angry (we don't actually know why) in his weird office of half-furniture, his scheme is in shambles and he is unsympathetic. He yells at Charlie and tells him he's a loser who gets nothing – but that's a position Charlie is good at. We've seen him fail gracefully before. Charlie believes he is sacrificing a chance to get his family out of poverty – the only thing that stops him is his moral compass, everything else in the film is signaling that it would be justifiable.
This is a perfect film, and every character stays true to their own nature. Through all the reveals and bizarre plot twists, characters begin and end as themselves. Charlie fails gracefully because in a morally ambiguous world he is a good person who considers others before himself. That is such a small, internal struggle it would have been eclipsed by the sensational antagonist and his world of candy-hued menace and elaborate schemes. We get those 40 minutes to establish a connection to humble Charlie and learn who he is. We see that his moral decision is truly his own. No one guides him to it, and we totally believe it.
I want to take a minute to contrast this script with one of the most over-rated scripts of all time: Star Wars.
Luke Skywalker has an un-earned journey from callow farmboy to savior of the galaxy. There are no moral dilemmas. We get such obvious stock melodrama characters they might as well be pantomime. Luke is not a sweet kid who we come to have sympathy for through his selfless actions, instead he is bored and petulant and feels entitled to more. He has a sort-of family (not his real family) who are just there to limit him, not love him. They die (off screen) and we watch their skeletons get thrown on a bonfire and feel… nothing. Those people just held him back from his destiny. We can't even consider them a Ma and Pa Kent who instilled a powerful boy with a moral compass, because Luke has no moral compass – he has an adventure compass. He is bummed later when Obi Wan is killed, and well, he wasn't family either and didn't teach Luke a moral compass, but he gave him a cool sword and took him to a bar. This is Luke's value system.
There is no discovery where Luke realizes he's in a bigger world because the movie already started at the bigger conflict. We saw a pretty angel-princess sass-mouth a nazi in a skull mask – zero ambiguity. Luke fights his way to rescue a damsel in distress but there's no surprise, we already know she's not what he believed. We are suppose to infer that Han Solo had some sort of change in his moral compass, but this also happens off screen, so it's as meaningless as the uncle and aunt's murder. Luke never sacrifices anything, and the whole galaxy is written in such broad strokes there is never any question what Luke is suppose to do. He is the "hero" because he shoots the thing. He is not a savior he's just the guy that threw the ball into the basket as the timer ran out.
Star Wars is so invested in how cool its main villain is, it forgets to make the protagonist into a hero. The first movie can be forgiven because it's all a fun topsy-turvy homage to bad serials, but by Rogue One we see Darth Vader appear from nowhere to serve some villain fan service. The villain is "Awesome Guy" while the protagonists are again jumping through an elaborate and noisy obstacle course that leads to an obvious and emotionally empty conclusion. All motivations are external and incidental to the characters. There's no reason one side is good and the other bad, they just are. Destiny in Star Wars is about who your parents were, not what kind of morals you have.