  We study the formation of spinning primordial black holes during an early
matter-dominated era. Using non-linear 3+1D general relativistic simulations,
we compute the efficiency of mass and angular momentum transfer in the process
-- which we find to be $\mathcal{O}(10\%)$ and $\mathcal{O}(5\%)$,
respectively. We show that subsequent evolution is important due to the seed
PBH accreting non-rotating matter from the background, which decreases the
dimensionless spin. Unless the matter era is short, we argue that the final
dimensionless spin will be negligible.
