Mirror effects in games is basically a resource hog. Remember it's not a 'real world' we see in the game. What we do see is a bunch polygons that are 'modelled' in a certain shape that it gives out the illusion of being a '3D' object, despite being '2D'.
This means that when you depict a 'mirror' and its 'reflection' in the game, the 'reflection' is actually a separate set of 3D model that looks exactly like the 'object in front of the mirror'. Imagine that you are in a Spitfire and two Bf109s are chasing you. When you look at the rear view mirror, what happens is the system must;
1) model your Spitfire
2) model the two Bf109s behind you
3) model a separate 'torso' part of the Spitfire that is viewed via the mirror
4) model two additional, teeney weeney Bf109s that is placed in the mirror area to make it look like a reflection (which in fact, is another set of 3D models)
5) add in all the other ambient 'stuff', such as the clouds, terrain features, and etc etc
...
Take for example IL2/FB.
It has rear-view mirrors as an option which you can turn on/off. My PC system, while certainly not state of the art, isn't too shabby at all - and yet the moment I switch on the mirror I start to feel the effects of all the extra resources being used on my PC. The frame rate drops instantly. The developers of IL2 certainly knew how much a mirror would effect the game, and that's why they even put a two-level detail option on the mirror, so when you set it to a lower level all the details are quite 'blurred' and the only thing you can really clearly see is the icons of the enemy plane. If you're playing a no-icon game in IL2, the only time you actually see an enemy plane in the mirror is when they are REAAAAAAALY close.
The thing is, when you start relying on the mirrors in IL2/FB, what happens is that anytime you actually see something in the mirror behind you, the enemy plane is already saddled up at a real close range and getting ready to blast you out of the sky. (Just like in real life!)
Hats off to the 'immersion factor' of IL2/FB. The game is so realistic, that even the mirrors are exactly as one would expect - practically useless!