A lower bound is derived for the expected number of observations required by an arbitrary sequential test which satisfies conventional conditions regarding the probabilities of erroneous decisions.
Wassily Hoeffding. "A Lower Bound for the Average Sample Number of a Sequential Test." Ann. Math. Statist. 24 (1) 127 - 130, March, 1953. https://doi.org/10.1214/aoms/1177729092