  A recent ultraviolet luminosity function (UVLF) analysis in the Hubble
Frontier Fields, behind foreground lensing clusters, has helped solidify
estimates of the faint-end of the $z \sim 5-9$ UVLF at up to five magnitudes
fainter than in the field. These measurements provide valuable information
regarding the role of low luminosity galaxies in reionizing the universe and
can help in calibrating expectations for JWST observations. We fit a
semi-empirical model to the lensed and previous UVLF data from Hubble. This fit
constrains the average star formation efficiency (SFE) during reionization,
with the lensed UVLF measurements probing halo mass scales as small as $M \sim
2 \times 10^9 {\rm M}_\odot$. The implied trend of SFE with halo mass is
broadly consistent with an extrapolation from previous inferences at $M \gtrsim
10^{10} {\rm M}_\odot$, although the joint data prefer a shallower SFE. This
preference, however, is partly subject to systematic uncertainties in the
lensed measurements. Near $z \sim 6$ we find that the SFE peaks at $\sim 20 \%$
between $\sim 10^{11}-10^{12} {\rm M}_\odot$. Our best fit model is consistent
with Planck 2018 determinations of the electron scattering optical depth, and
most current reionization history measurements, provided the escape fraction of
ionizing photons is $f_{\rm esc} \sim 10-20\%$. The joint UVLF accounts for
nearly $80\%$ of the ionizing photon budget at $z \sim 8$. Finally, we show
that recent JWST UVLF estimates at $z \gtrsim 11$ require strong departures
from the redshift evolution suggested by the Hubble data.
