Histograms of (left) residual norms and (right) iteration steps of PCG runs performed on simulated data on SP, LP, and VLP in the top, centre, and bottom panels, respectively. Blue and orange bars show the histogram related to PCG runs with M_BD and with M_2l applied with the simplified approach, respectively. In the top panel we also show PCG runs applied with the active approach with ϵ_Arn = 10⁻⁶ (10⁻¹²) as red (green) bars.