Not only are they better placed in the common code, but they are also both per-viewpoint and not per-scene, so this is a far more suitable place and avoids saving and restoring them in the portal code.
On a fast and modern graphics card this is a lot faster than doing it per subsector but it may not be without drawbacks on older hardware so it will require some testing on older hardware.
For me Frozen Time's view over the bridge went from 46 fps to 51 fps with this change, the time saved was roughly 2 ms.
