Best Supporting Actress, which has no minimum screentime requirement. There's no such thing as Best female performance at the Oscars.
Exactly. Judi Dench and Beatrice Straight have both won for shorter roles, and Judi Dench's case a less important role. I don't with screentime matters, it's what the actor does with it, if anything it's even more impressive to make such a big impact in such little time.