Surely what most pilots mean by "groundspeed" is the distance over the ground (i.e. the Earth's surface) covered in unit time. Therefore, the fact that you may be at altitude and cover a longer distance by virtue of your distance from the centre of the Earth is irrelevant. Otherwise your groundspeed becomes a function of altitude.
If you want to be pedantic with the physics, nobody seems to have mentioned Einstein's relativistic effects of both General and Special Relativity.
The general relativistic effect will mean that because gravity is slightly less up there the aircraft clock will run faster than it would do on the ground, and this is countered by the special relativistic effect of high speed motion which means that the faster you go, the more time slows down.
It is quite possible with modern atomic clocks to demonstrate these effects in aircraft.
So every time you go flying, so long as you don't fly too fast, you are actually aging slower than you would do on the ground.
