As you might know speed is a very importand issue for 3D engines, a good start for performance is to write highly optimized code. Here’s an article about how John Carmack uses a magic constant (0×5f3759df) to calculate an inverse square root. Article link

However interesting, the credits for this magic constant might not be Carmac’ but someone else’ as you can read in this post on beyond3D. A e-mail from the man himself actually confirms this.