With stereo off, the FPS reads around 60fps (again there it is a bit trick to read as the value is obviously updating very quickly so it is hard to say if it is 60fps or above 60fps). The timing seems quite steady at 16ms. The display in the Vive goes blank with stereo off, but I assume this is normal.
With stereo on, I get the much more dramatic changes in fps:- it seems to be anywhere between 50 and 70fps (perhaps as low as 40). This is with the headset not even moving. The timing also varies much more erratically too.