When more than two cameras my screen lags.

I made a mini map with a little help of a video. It involved a camera over my head projecting a mini map on my game view. The mini map works fine but when I play my screen lags. I thought it was because the two cameras were rendering at the same time. If anyone has any idea what is going on please tell me what and if you can please tell how to fix this.

The cheapest thing to do would be to make a texture of your map layout, set this on a plain, define the plane on a certain layer, and make your camera only render that layer. Then the minimap camera will only render the texture and ignore all the polygons in your scene.