Exploration of the Moon and Mars calls for robots that are increasingly capable in regolith, or granular soil. Beyond traversing and avoiding entrapment, future robots will excavate and process regolith as a resource. This work distinguishes concerns governing the performance of regolith operations, based on load-haul-dump tasks motivated by in situ resource utilization and lunar outpost site work. Payload ratio (mass of regolith payload capacity normalized by robot mass) and driving speed are identified as key parameters governing the productivity of small site work robots. Other parameters, such as number of wheels, are not as important. Experiments with a small robotic excavator and task-level simulations (for which a modeling framework is described) determine the relative sensitivity of productivity to changes in these variables. These findings provide direction for the development of future lightweight robotic excavators.