It you select the variable time step option in SWMM 5 then the program will compute the CFL time step for each link based on the ending system variables in the last time step based on the following steps.  The smallest value of t is used for each time step but often the same small set of links will be the controlling time for the whole simulation.  In the example shown below link  1570c is controlling the time step 83 percent of the time.  The link time step is usually the controlling time step.