Premade adventures probably don't save me a lot of time, but allow me to spread it better. I usually read an adventure completely long before running it. The, with the conceptual information in my head, I can do my preparations in small amounts whenever I want to and time permits. If the stuff is really interesting, I also think a lot about possible variations, extensions and hooks suitable to my game.
When the time comes to actually prepare the adventure, most of the brain work is already done and I have a pretty complete picture of my version of this adventure in my head.