Recently I tried Sauerbraten, a free first-person shooter. (It's like Quake, with an emphasis on network play and in-game map editing.) I got schooled, of course. And in addition to the lesson in how (not) to dodge, I got a lesson in user interfaces.
In any such game, your current hit points are one of the most important pieces of information. Your remaining ammunition is nearly as important. Therefore Sauerbraten shows both numbers in big numerals in the corners, where you can see them at a glance.
But a glance takes too long. Sauerbraten is a fast and twitchy game, and you can't afford to take your eyes off a fight, even for a fraction of a second. Looking at the number and back to the fight takes perhaps a third of a second, but that's enough time to lose track of an opponent, or to fatally delay your reaction to a shot.
So hit points, ammo, and other information needed in the heat of battle should be displayed in a form that can be read with peripheral vision, so you don't need to move your eyes. A large bar graph - at least half as tall as the screen - works fine.
One of the rules for presenting information more efficiently is to let the users navigate with their eyes, not their hands, because the eyes are faster. In general, it's faster to look at something that's already visible than to push a button to display it. An efficient interface minimizes the number of places the user's eye needs to look, just as it minimizes the number of operations their hands need to do. But in the real-time extreme of a game, even one glance is too many. The most urgent information needs to be visible without moving the eye, because the eye is too slow.