We consider the problem of maximizing a monotone submodular function subject to a knap- sack constraint. Our main contribution is an algorithm that achieves a nearly-optimal, 1−1/e−ǫ approximation, using (1/ǫ)O(1/ǫ4)nlog2 n function evaluations and arithmetic operations. Our algorithm is impractical but theoretically interesting, since it overcomes a fundamental running time bottleneck of the multilinear extension relaxation framework. This is the main approach for obtaining nearly-optimal approximation guarantees for important classes of constraints but it leads to Ω(n2) running times, since evaluating the multilinear extension is expensive. Our algorithm maintains a fractional solution with only a constant number of entries that are strictly fractional, which allows us to overcome this obstacle.