Sentence:  An artificial kidney these days still means a refrigerator-sized dialysis machine. Such devices mimic...real kidneys.... But they are nothing like as <WSD>efficient</WSD>, and can cause bleeding, clotting and infection—not to mention inconvenience for patients, who typically need to be hooked up to one three times a week for hours at a time. 
Meaning of the word marked with <WSD> tags:  making good, thorough, or careful use of resources; not consuming extra. Especially, making good use of time or energy