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Near extremal Kerr black holes are subject to the Thorne limit $a<a^*_{\rmlim}=0.998$ in the case of thin disc accretion, or some generalized version ofthis in other disc geometries. However any limit that differs from thethermodynamics limit $a^* < 1$ can in principle be evaded in otherastrophysical configurations, and in particular if the near extremal blackholes are primordial and subject to evaporation by Hawking radiation only. Wederive the lower mass limit above which Hawking radiation is slow enough sothat a primordial black hole with a spin initially above some generalizedThorne limit can still be above this limit today. Thus, we point out that theobservation of Kerr black holes with extremely high spin should be a hint ofeither exotic astrophysical mechanisms or primordial origin.

Evolution of primordial black hole spin due to Hawking radiation