Both procedures imply an exchange of information between processors at every time-step, which can be very penalizing when using a large number of processors. Quoting  “A major obstacle to achieving significant speed-up on parallel machines is the overhead associated with synchronizing the concurrent processes”. One way to overcome this delay problem was invented in the seventies with the concept of asyncronous algorithms, . An excellent review can be found in . In that context, amounts of information are sent without waiting for the request. However convergence is weakened, if not destroyed.