laconas: I don't think anybody really knows.
It's the speed of light. Virtual photons fly around at their accustomed speed, creating the electrical field in the space around them. The speed of gravity is thought to be the same, mediated by a so-far undetected particle called the graviton.
I know what a virtual image is. I have been an optical engineer in a prior incarnation. But I do not know what a virtual photon might be.
For the speed of gravity determination they are using massive but highly sensitive devices. If something heavy moves, like a supernova, they might be able to get a measurement. I think they are waiting for a suitable event so they can get some numbers. Until then they are detecting neutrinos and doing Aetvos experiments with artificial satellites. Maybe we will be the generation that gets a clue.