OK
I ran the ViewingDaleTestApp a total of 18 times (3 times in each condition), with results as follows (in frames per second):-
With CD off
Hardware acceleration - 314, 304, 303
Software rendered - 15, 15, 15
With CD on and hidden in System Tray
Hardware acceleration - 5, 4, 5
Software rendered - 4, 8, 5
With CD on and showing on screen
Hardware acceleration - 76, 77, 76
Software rendered - 13, 13, 12
Very strange, but very consistent.
I don't know enough to even try to explain it.