Coasting works if the person is just about fulfilling their responsibilities.
From Oxford dictionaries:
Proceed without making much effort: "Colchester coasted to victory"
From Miriam-Webster (relevant meaning in bold):
to move forward using no power or very little power
to move downhill by the force of gravity
to progress or have success without special effort
... "He was accused of trying to coast through school"
... "The company is coasting on its good reputation"
This meaning is (I believe) an analogy to a bad practice in driving: letting the car just roll along with the clutch off, with no control or power from the engine. It is going forward, but the driver is neglecting to properly control the car.
As a commenter pointed out, "going through the motions" might be too strong since it implies not actually performing a task or achieving results. Coasting is more appropriate if they are getting adequate results, but via dangerously little effort. If someone says "You're coasting, Bob", and you reply "But I met my targets", that wouldn't logically contradict the accusation - coasting is a mild criticism that already implies nominal success or progress.