Well, they'd have to take the film and do minimal processing on it, and yes it was pretty time consuming. They would sometimes have different cameras just for dailies.

However, starting in the 1960s, they started using videotape alongside film cameras for the dailies. This greatly simplified the process. The first movie to do this was The Party starring Peter Sellers and directed by Blake Edwards, which was released in 1968.

(And I know this because it's one of my favourite movies (; ).