In this thread, people who don't understand how 3D works.
You aren't rendering twice as many polygons as 2D mode, that would be terribly inefficient. You're using the same model, and you're loading the same textures, you're just taking 2 viewpoints instead of one. You will have to render more objects because the slight perspective difference gives you slightly different FOV, etc, but you aren't doing twice the work.
Yes, this is more of a strain then a single camera, but it's definitely not DOUBLE the work... if it did, 3D modes would always half framerate at a minimum (anyone that has done 3D games on PC know this isn't the case).
As for the "120fps" thing, that's BS, the 3DS isn't "flipping" between two images every second, it's displaying the entire image "side by side", so the screen refresh rate will ALWAYS be capped at 60. Interlaced in any form would make zero sense on an LCD display, it would just make the image stutter.