There’s nothing wrong with detailing your world or campaign down to the smallest element (which can be very enjoyable in its own right), but it’s easy to get sidetracked and spend too much of your precious prep time on stuff that won’t see the light of day. I’ve certainly done my fair share of this in past games!
If you want maximum value for your prep time, focus only on what you know will come up — and make those elements as cool as possible. Sketch out the stuff on the periphery if you have time, and don’t spend any time on things beyond that periphery.
For example, if the PCs are going to be infiltrating a secret government facility, you might be tempted to write up the history of that facility. Unless your players will care, ignore that temptation. Instead, use that time to spruce up your map, come up with a really memorable NPC or otherwise work on something that’s guaranteed to enhance your game.