Massive performance drop when placing same mesh in same position

Hi everybody,

due to a bug in my application I placed several (equal) meshes in exactly the same position. This caused a massive framerate drop (from about 500 fps to 30 fps). Another issue was that the closer I get to the multiple meshes the worse the framerate became. When I moved away for about 50 m the behaviour was almost normal again. The error is fixed now and everything is back to normal again but I don't understand why this bug caused such a behaviour. Could someone elaborate on this? I just like to understand what happened. (It obviously has something to do with shadows and culling.)

Thank in advance!

G.