Rapid thermal processing (RTP) has found continually increasing use for oxidation, silicidation, CVD, and other steps in microelectronic fabrication. Kinetic effects in rapid thermal processing (RTP) are often assessed using the concept of thermal budget, with the idea that low thermal budgets should minimize dopant diffusion and interface degradation. Some definitions of budget employ the product of temperature and time (T-t). In previous work, we have shown that this definition for budget often leads to qualitatively incorrect conclusions regarding heating program design. However, other definitions of budget employ the product of diffusivity and time (D-t), where the diffusivity describes unwanted diffusion or interface degradation. Here we show that minimization of D-t by itself is insufficient to kinetically optimize a heating program; account must be taken of the relative rates of the desired and undesired phenomena. We present a straightforward but rigorous method for doing so.