I just want to make a little square box a mirror, that wont hurt my fps, right?
Only one side of the box may have the reflective glass texture because of engine limitations. Additionally, only one Mirror is allowed per leaf. More of them create a strange and bad looking HoM (Hall of Mirrors - that what happens when you fly into the void in spectator mode) effect.
And (yeah, even more limitations ^^) you are not allowed to use water in the leaf of that mirror. If you want a "cheap" mirror, try using one of the "chrome"-textures with a cubemap with maximum size. Doesn't look as good, but causes fewer bad effects.