Inefficient things waste time, money, or other resources. Your old inefficient dishwasher uses much more water and electricity than a new model would. Shame on you.
Inefficient businesses have trouble making money and growing, and an inefficient city bus system might transport just a few passengers at a time. If your car is inefficient, it gets terrible gas mileage and is expensive to drive. If something is efficient, it's productive and inexpensive. Inefficient has the opposite meaning, which is clear from the "not" prefix in-.