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There is annual quota for greencards in each category. For e.g.
within Employment type EB1 has certain number, EB2 has certain and so on. And again within a category, no single country can exceed so many visas (this include dependents). Thatswhy generally there is a backlog for people who belong to China & India.
It definitely affects if one center is more efficient than others. But the way Priority dates are progressing, it may not be a huge concern as of now.