We present the first measurements of the dynamic response of InGaAs/InGaAsP multiquantum-well lasers which clearly show the effect of the carrier transport phenomena on the modulation response. Using our parasitic-free optical modulation method, we separate an intrinsic RC-like component in the frequency response from normal resonance behavior. The bandwidth is strongly limited by this low-pass rolloff that can be described by a structure-dependent transport time. It is shown that, therefore, the K factor may not be a reasonable quantity for evaluating the ultimate bandwidth.