To efficiently deliver computing power, various requirements are essential for DC-DC converters, but fulfilling these requirements involves challenging tradeoffs. For instance, to achieve better efficiency, the converter with high-voltage high-current and poor figure of merit (FoM) devices should be operated at lower frequency for high input voltage applications. However, this design inevitably sacrifices the converter's transient response performance. Therefore, a solution that can decouple all these requirements are desirable. Recent hybrid DC-DC converters use dual inductor to increase the current slew rate for transient improvement, however the improvement is limited by only 2x [1]. Here, in this work, we propose a fast-slow two-module DC-DC solution, which is an optimum solution for the efficiency-transient speed tradeoff and for 2.5D/3D integration [2]. As shown in Fig. 1 (bottom left), the proposed solution utilizes a low-frequency (LF) buck converter to supply high current and a high-frequency (HF) buck converter to achieve fast transient response. Fig. 1 (bottom right) show the system integrated package. The LF buck converter is located on the back side for vertical power supply, while the HF buck converter is integrated with processors for fast transient performance.