I am working on a fairly simple scene (80k verts) that is mostly made up of simple objects with some subsurf. Normally, my viewport performance is just fine. However, when I hit numpad 0 to view the scene through the active camera (not rendering) my performance takes a massive hit and begins to lag. Disabling subsurf and solidify on objects helps, but does not solve the problem. (This drops the scene to 53k verts.)
This performance hit is strange as I have not seen it before, even in earlier versions of the same project file, and I normally have no problem handling much more complex scenes (GTX Titan GPU.)
What could be making it change merely by being in the active camera? The amount of the model visible is the same in both cases. The viewport and actual camera have the same lenses and clipping distances.
Update: I did some further testing. I created a second camera , and it does not have the performance drop when I use it. The first still does. They have the exact same default settings. I even tried changing the first camera to the second's data, and the second to the first, and still the performance hit only takes place in the first camera, regardless of camera data. So it's something to do with the object, not the camera settings. But I can't find any differences. I also tried duplicating the first camera both with ctrl+D and alt+D, and neither of the duplicates have the problem.
