To utilize seabed mineral resources, which can be found at the bottom of the ocean, it is necessary to collect and analyze samples. Therefore, we intend to develop a seafloor robotic explorer that can excavate and sample the seafloor soil. In a previous study, we developed an underwater drilling robot. This robot could excavate into the ground while underwater. However, this robot had a problem. The actual time taken to complete the excavation was longer than the expected time to complete the excavation. Therefore, the goal of our current study is to reduce the drilling resistance and therefore shorten the completion time. We adjusted the penetration speed and the rotational speed of the drilling robot. As a result, we succeeded in reducing the drilling resistance, which both lowered the excavation energy by 28% and significantly reduced the completion time.