From Fig. , we can see that both the real and ideal speed-ups are quite far from the limit speed-ups (up to a factor 3 of difference). This is simply a consequence of the fact that the condition, t high t low is not satisfied here. For δt = 10 a.u. t high 5. The real and ideal speed-ups are much closer from each-other and with an MTS factor between 3 and 6 it seems that reliable results could be obtained with a speed-up factor ranging from 1.5 to 2.5. The number of triggered high level calculations does not seem to affect the speed-up significantly since, in the considered calculations, the maximum averaged frequency of triggered high level calculations [X in eq. ( )] is equal to 0.026, which corresponds to a triggered call every 38 inner step. Most of the differences between the real and ideal speed-up in Fig. can thus be attributed to the convergence overhead discussed above.