Observations and modeling indicate that galaxies convert only a small fraction of their available baryons into stars, with a strongly mass- and redshift-dependent “efficiency” that peaks around Milky Way–sized halos and declines at both lower and higher masses (Behroozi et al. 2013; Conroy & Wechsler 2009). ΛCDM-based galaxy formation models can broadly reproduce these trends by tuning feedback and gas accretion prescriptions, but they struggle with the detailed shape, evolution, and large intrinsic scatter of star formation efficiency at fixed halo mass, especially at very high redshift where JWST finds surprisingly efficient early systems, revealing tension between simple halo-driven efficiency rules and the complex, environment-dependent behavior seen in real galaxies (Behroozi et al. 2013; Silk & Mamon 2012).