Building on the flexibility of open-source RISC-V- based CPU designs at the register-transfer level (RTL) we deliver a characterization study that is not feasible on commercial CPUs. We identify the major power-consuming hardware structures by focusing on SonicBOOM’s out-of-order (OoO) microarchitecture across three design points of increasing aggressiveness. By introducing and employing the SimPoint methodology on a diverse set of workloads, we shed light on the relationship between microarchitecture and energy efficiency of BOOM, which is the highest-performance CPU design in the public domain. Our analysis highlights the Branch Prediction and the Instruction Scheduler Units as the most power-intensive components. We evaluate the energy efficiency (performance per watt) of the three design configurations of BOOM and conclude that the smallest of the three OoO cores, while being the slowest, prevails. The proposed experimental flow can be used to evaluate any CPU design using arbitrarily large workloads due to the effective use of the SimPoint methodology we introduce in Chipyard - in our case offering a 45-fold reduction of simulation time. Our findings, encompassing 8 key takeaways, can assist microprocessor designers in optimizing energy efficiency by addressing major power contributors.