The space-charge capacitance of the forward-biased junction has been found to play a major role in (i) the apparent rise time of the emission from small-area, high-radiance LED's and (ii) the apparent turn-on delay of stripe-geometry DH-structure laser diodes. For a zero-bias capacitance of 200 pF, a typical value for such devices made by oxidemasking techniques, the measured rise time of an LED that is fully turned on and the turn-on delay time of injection lasers may be as much as twice the limitation imposed by the spontaneous carrier recombination time. A proposed method to reduce these delays by preshaping the driving pulse is analyzed, and a reduction of the delay by a factor of 2 or better is predicted. These results are in agreement with experiments.