Fraction of halos with a star formation efficiency value (f_*^sphinx) below the value shown on the horizontal axis from the SPHINX²⁰ simulations, for the mass bins marked, at z = 5 (left panel) and z = 9 (right panel). The star formation efficiency is defined as the ratio between the mass of stars younger than 30 Myr and the cold gas mass; if a halo has no cold gas mass, we define its star formation efficiency to be f_*^sphinx = 0.