Even if P = NP, the search for efficient polynomial time algorithms will be the key. For e.g., the problem of determining whether a given number n is prime/not was thought to be in NP, until Manindra Agarwal showed that "PRIMES is in P" (it's called the AKS algorithm). Its asymptotic complexity (as n -> infinity) is O(polylog(n)), but it is not widely used in practice, it is still "slow".