I think it's less about the speed of the evolution per se versus the different selection pressures. For example black people are far more likely than other races to have alleles for sickle cell anemia, because while two recessive copies produces sickle cell anemia, a single recessive allele produces only minor impairments in blood cell efficiency but massive improvements in malarial resistance.

(Note I prefer using the term race rather than ethnicity when talking about these thinks, because the concept of race correlates a bit better to genetics than ethnicity. Both are arbitrary social constructs, as everything in life is, but ethnicity has more to do with identity than it does raw physical similarity)