We present a model that predicts the saturated single-hop performance of IEEE 802.11 ad hoc network in the face of the physical capture effect under a block-fading Rayleigh channel. Based on an estimate of the frame delivery rate and a fixed point iteration algorithm, we devise an efficient control on the rate of convergence and a wise selection of the interferer sets that significantly affect the final results, so as to overcome the explosive time complexity of the iteration algorithm. These enhancements achieve up to two orders of magnitude speedup for large ad hoc networks with minimal and controllable prediction error, so that large scale Monte Carlo experiments for various distributions are made possible.