Another possibility is one that I call, "Fat Instants." In this model, each instant actually has some "width" ("length") and they can overlap. The greater the overlap, the more change that can occur "during" that overlap. An overlap of 100% implies an infinite amount of possible change -- it caps how fast something can travel and hence how fast things can change (since all change is due to movement). This is synonymous with infinite speed -- i.e., the speed of light. This would also explain why there is a maximum speed possible.