If two processes make the same safe product, one using less energy and material is more efficient in those respects. Economists also ask whether resources could be rearranged to make some people better off without making anyone worse off. The question depends on which costs and benefits are counted.
A process that looks cheap to its operator may impose costs on neighbours through pollution or delay. Good comparisons state the goal and include relevant consequences before calling an outcome efficient. An efficient result is not automatically fair.