The computational effort and run time is directly proportional to the number of elements in the model. If you have a million-element model that is predicted to run in 10 hours, and you coarsen the mesh so that you have a half-million elements, the run time will decrease by more than a factor of two.
There are two factors in play. First, you have reduced the total number of elements and hence reduced the computational effort. But, second, by making the mesh coarser you increase the element sizes and therefore the time step size, so fewer time steps are required.