Some characters that had around 15 minutes or less in a film but had a huge impact on it:
The Wicked Witch of the West (Margaret Hamilton) in The Wizard of Oz (1939)
'Boo' Radley (Robert Duvall) in To Kill a Mockingbird (1962)
Fantine (Anne Hathaway) in Les Misérables (2012)
Maybe (not sure if any are 15 minutes or less):
Blofeld (Donald Pleasence, Charles Gray and others) in various James Bond films.
@nealfig
Hannibal Lector (Anthony Hopkins) in Silence of the Lambs (1991)
Lynn Bracken (Kim Basinger) L.A. Confidential (1997)
@Animeraider Definitely Hannibal Lector! I can't remember L.A. Confidential but will watch it again.