People Form Judgments in 100 Milliseconds, and Stick to Them
The most-cited study in this space is Willis & Todorov (2006), published in Psychological Science. Participants formed stable judgments about traits — including likeability, trustworthiness, competence, and aggressiveness — after only 100 milliseconds of exposure to a face. Crucially, those rapid judgments correlated highly with ratings made under no time constraints, and longer viewing times did not significantly change the evaluations.
In other words: the first impression doesn't just matter — it's nearly impossible to override.