Things grow in fast movies
A movie often has to fit a few seasons or years into a few hours. Because of this, you might see a bleak winter day transform to spring within a matter of seconds, just to keep the plot going. Nick uses some hyperbole to compare the rapid growth around him to the movies.
“Fast movies” may also refer to time-lapse movies like the one above (of “things growing fast”). Time-lapse photography was in fact pioneered in the early 20th century with many early experimenters focusing on the growth of biological phenomenon over time. By the 1920s when The Great Gatsby was published, these experiments had become part of feature films.
To help improve the meaning of these lyrics, visit "The Great Gatsby (Chapter I)" by F. Scott Fitzgerald and leave a comment on the lyrics box