Speedup S(LM) of the formal solution of the RT equation due to simultaneous parallelization in radiation frequencies and domain decomposition. Filled circles: data calculated for various values of L and M using the OCAS cluster. Diamonds: data calculated using the LaPalma supercomputer. We note that because of non-optimal spatial and frequency decompositions (i.e., not all the CPU cores treat exactly the same number of grid points and/or frequencies), small oscillations around the nearly linear trend appear. The speedup in both data sets is normalized to the serial time T₁ corresponding to each of the used computers.