i think the frame rate is the same when its not lagging. It starts lagging as soon as i walk in range of the second camera which is rendering to the texture. Like almost freezes less than 1 fps. Im on Windows 7 64 bit and using a Radeon HD 5870. Im not entirely sure when it started happening I used to have a sphere that had texture rendered from the same camera as the display in windowed mode (only had 1 camera at this time) and it didn't lag at all, but then i started testing stuff in fullscreen and didn't realize that at some point, my windowed mode got messed up.
However, I noticed if I removed some of the objects in my world the lag vanished. Alternatively, if I ended the windows process svchost (it was using 200mb of memory), the lag would basically dissapear as well until svchost was automatically started back up. Not sure why that happens, I did notice that when svc host is terminated, the start menu bar on Windows 7 becomes a solid color instead of being transparent so I dunno if thats related to anything.