In this work, we analyze the optimum expected end-to-end distortion (EED) in delay-constrained wideband multiple-input mulitple-output (MIMO) systems. We prove that the existence of frequency diversity benefits the EED in delay-constrained systems though it does not impact the ergodic channel capacity. For further analysis, we derive the closed-form expression of the optimum asymptotic expected EED, comprised of the optimum distortion exponent and the multiplicative optimum distortion factor. We present that, the optimum asymptotic expected EED decreases monotonically with frequency diversity order, but EED does not vanish with infinite frequency diversity. We also investigate the impact of spatial correlation on EED in wideband systems. The theoretical results in this paper can be guidelines for practical wideband system design.