We present a simple and general model for a small-signal admittance of double-barrier resonant-tunneling-diodes (RTDs). The model accounts for both the effect of traveling electrons in the RTD collector spacer and the dynamics of the electron charge in the RTD quantum well. Using the model, we evaluate how the electron travel time through the collector influences the maximum oscillation frequency of state-of-the-art RTDs. The analysis shows that the impact of the traveling electrons is more pronounced in RTDs with high current density, and that a shorter collector spacer is advantageous for high-frequency RTD oscillators. However, if electrons travel ballistically through the spacer, their effect is nearly negligible.