I have written about the characteristic (modal) rate for a mode of travel. This rate provides a factor for converting spatial into temporal measures and vice versa. The characteristic rate is the maximum (free flow) rate, though it could be the minimum rate. A characteristic rate is independent of any and all particular rates in a mode.
The speed of light in a vacuum is widely considered the maximum speed for signals. What is important is not so much that it is the speed of light as that it is the speed that is the supremum for the speeds of all bodies with non-zero rest mass. It so happens that the photon travels at this speed, called c.
I wrote recently about a lower limit on speed equal to the inverse of the speed of light. This is better described as the infimum of the speeds of all bodies with non-zero rest mass. Is there a particle that travels at this speed, 1/c? It might be a “lygon” for twilight particle (from lygo, Greek root for twilight).