Oculus Rift automatic down-sampling and Duplicated displays

This came up in the Half-Life thread, but it isn't specifically a Half-Life question, so I wanted to start a new thread for this issue.

The Rift exposes a bunch of different valid resolutions to Windows even the panel itself is 1280x800 and only 1280x800. It goes up to 1920x1200 (which is also 16:10). Not sure how far it goes down.

After some research (and measuring) it seems to us that the down-sampling occurs in the Rift converter box and that it doesn't introduce significant latency. The way we measure latency (with a high speed camera and some blinky LEDs) we have a minimum resolution of about 3ms, so a 0ms measurement means something less than 3ms. This is all just display latency and doesn't include any measurable rendering time or tracking. This was all under DX11, in case that's significant. (The Source games actually run under DX9.)

Full screen with the display extended to the Rift (not duplicated):
1280x800: 0ms
1920x1200: 0ms

Windowed without Aero and the display extended to the Rift:
1280x800: 10ms

When you switch to duplicating the desktop monitor and the Rift things get strange. The tester runs at something like 1000Hz, and it appears that Windows is actually dropping most of the frames and never sending them to the display at all. This happens in both Full-screen and Windowed, but the final swap in windowed is slow enough that fewer frames get dropped. It dropped so many frames that measuring the latency was actually hard to do.

So the verdict is:
1) Letting the Rift scale down the frames is more or less fine
2) I need to switch the Source games to use full screen instead of windowed to save 10ms
3) Cloning your main display is bad and is probably causing extra latency.