Why does gas form stars at only one percent per free-fall time?
In plain words
If gas clouds simply collapsed under their own weight, galaxies would form stars about a hundred times faster than they do. Turbulence, magnetic fields and the push of newborn stars all slow it down, but which one sets the value is unknown.
Precise statement
Molecular clouds convert gas into stars at efficiency per free-fall time eps_ff = SFR t_ff / M_gas ~ 0.01, with integrated cloud efficiencies of $\sim 1-10\,\mathrm{percent}$ before dispersal. Explain the value and its scatter as a function of cloud surface density and virial parameter, separating the roles of supersonic turbulence, magnetic support and stellar feedback (photoionization, winds, radiation pressure, jets, supernovae).
What would settle it
Simulations with all feedback channels that reproduce the measured eps_ff distribution across cloud populations, plus observations that measure eps_ff and its dependence on surface density in many galaxies.