  Recent JWST observations suggest an excess of $z\gtrsim10$ galaxy candidates
above most theoretical models. Here, we explore how the interplay between halo
formation timescales, star formation efficiency and dust attenuation affects
the properties and number densities of galaxies we can detect in the early
universe. We calculate the theoretical upper limit on the UV luminosity
function, assuming star formation is 100% efficient and all gas in halos is
converted into stars, and that galaxies are at the peak age for UV emission
(~10 Myr). This upper limit is ~4 orders of magnitude greater than current
observations, implying these are fully consistent with star formation in
$\Lambda$CDM cosmology. In a more realistic model, we use the distribution of
halo formation timescales derived from extended Press-Schechter theory as a
proxy for star formation rate (SFR). We predict that the galaxies observed so
far at $z\gtrsim10$ are dominated by those with the fastest formation
timescales, and thus most extreme SFRs and young ages. These galaxies can be
upscattered by ~1.5 mag compared to the median UV magnitude vs halo mass
relation. This likely introduces a selection effect at high redshift whereby
only the youngest ($\lesssim$10 Myr), most highly star forming galaxies
(specific SFR$\gtrsim$30 Gyr$^{-1}$) have been detected so far. Furthermore,
our modelling suggests that redshift evolution at the bright end of the UV
luminosity function is substantially affected by the build-up of dust
attenuation. We predict that deeper JWST observations (reaching m~30) will
reveal more typical galaxies with relatively older ages (~100 Myr) and less
extreme specific SFRs (~10 Gyr$^{-1}$ for a $M_\mathrm{UV}$ ~ -20 galaxy at
z~10).
