An ounce of performance is worth pounds of promises.
I'm so fast that last night I turned off the light switch in my hotel room and was in bed before the room was dark.
When dealing with 3D worlds, performance is nearly always an issue because of the intensive mathematics required for even simple scenes. If all you want to render is a bouncing cube, then not to worry, but if you want to display the universe, you'll always be concerned about performance.
Up until now, the exercises have been presented in a way to be reasonably clear (I hope) but not necessarily efficient. And unfortunately, efficient code is rarely the clearest and most easily understood. So now, we're going to start ...