What signal are you referring to? what medium would that be? it really depends on the medium your using since signals travel through mediums at rates relative to the speed of light (Velocity Factor) and each medium varies. The highest possible speed is of course the speed of light and we only operate at fractions of that, but science is making great progress. Also, the processing these signals would also add to the latency due to post signal processing, ie. light has to be converted to electrical signals again as compared to a copper wire that could use the signal directly. Another factor to put in mind is the bandwidth that a medium can carry, for large amounts of data, Fiber optics win since it can support very high frequencies that a copper wire cannot.
I'm going a bit off topic, but to in short, the speed of light is the ceiling but signal speed propagation is only a part of a larger system of communications.