Schematic diagram to illustrate the standard SB-QSO evolutionary sequence. In the first two panels, the blue ellipse and yellow star represent the galaxy and the central SMBH, respectively; the light and dark grey areas refer to inflowing gas and dust. Dark blue clouds with yellow outlines indicate outflows. In the last panel, the yellow star and ellipse represent the SMBH and the QSO emission. In the first phase (left), the central regions are fuelled by huge amounts of gas and dust, and the SMBH is highly obscured. These systems, originating from gas-rich galaxy mergers, are commonly identified as SMGs, and are associated with star formation efficiencies of ∼100 ÷ 200. The SFE of CT/highly obscured QSOs is mostly unknown. These systems, if tracing the same evolutionary stage, should have SFEs similar to those of SMGs. In the last phase (right), the system is revealed as an unobscured QSO (bright enough to easily outshine its host galaxy). In this phase its gas reservoir is expected to be strongly depleted. The SFE measured in optically luminous QSOs can span a wide range, from a few 100 to ∼2000. In the transition phase (middle), galaxy-wide outflows start to remove the dust and gas reservoirs (which only partially obscure the nuclear regions). We observe these systems as obscured QSOs (generally associated with signatures of outflows). For these systems, we naively expect to observe intermediate SFE between SMGs and unobscured QSOs.