  Over the past decade, the existence of a substantial population of optically
invisible, massive galaxies at $z\gtrsim3$ has been implied from mid-infrared
to millimeter observations. With the unprecedented sensitivity of the JWST,
such extremely massive galaxy candidates have immediately been identified even
at $z>7$, in much larger numbers than expected. These discoveries raised a hot
debate. If confirmed, early, high-mass galaxies challenge the current models of
galaxy formation. However, the lack of spectroscopic confirmations leads to
uncertain stellar mass ($M_{\star}$) estimates, and the possible presence of
active galactic nuclei (AGN) adds further uncertainty. Here, we present the
first sample of 36 dust-obscured galaxies with robust spectroscopic redshifts
at $z_{\rm spec}=5-9$ from the JWST FRESCO survey. The three most extreme
sources at $z\sim5-6$ ($\sim$1 billion years after the Big Bang) are so massive
(log$M_{\star}/M_{\odot}$ $\gtrsim11.0$) that they would require, on average,
about 50% of the baryons in their halos to be converted into stars -- two to
three times higher than even the most efficient galaxies at later times. The
extended emission of these galaxies suggests limited contribution by AGN. This
population of ultra-massive galaxies accounts for 20% of the total cosmic star
formation rate density at $z\sim5-6$, suggesting a substantial proportion of
extremely efficient star formation in the early Universe.
