Slim disc efficiency. The solid lines indicate model efficiencies for a number of mass accretion rates. The black line corresponds to the standard thin disc efficiency. For HLX-1 we derive effective efficiencies from the fitted disc luminosity of the XMM-Newton spectrum and the observed unabsorbed 0.3−10 keV disc luminosity for selected inclinations (i = 0°, 30°, 50°, 60°, 70°, and 80°). The dashed line highlights the spin evolution of the effective efficiency for a disc with i = 60°. Error bars include the uncertainty in black hole mass and disc luminosity.