The problem of the critical slowing-down affecting local processes is of fundamental importance in the study of second-order phase transitions (like ferromagnetic transition in the Ising model), as increasing the size of the system in order to reduce finite-size effects has the disadvantage of requiring a far larger number of moves to reach thermal equilibrium. Indeed the correlation time τ τ usually increases as L z with z ≃ ≃ 2 or greater; since, to be accurate, the simulation time must be t ≫ ≫ τ τ , this is a major limitation in the size of the systems that can be studied through local algorithms. SW algorithm was the first to produce unusually small values for the dynamical critical exponents: z = 0.35 for the 2D Ising model ( z = 2.125 for standard simulations); z = 0.75 for the 3D Ising model, as opposed to z = 2.0 for standard simulations.