GDP / (average annual labor hours * number of people working in some way)?

GDP is a pretty rough metric for efficiency, but it's among the better ones that are collected somewhat uniformly.

"average hours worked * number of people working" is an approximation of total of person-hours worked (approx. because both numbers are probably cleaned up a bit in incompatible ways)

The result would be the value a person in the country contributes to the GDP every hour (on average), which should serve as a reasonably proxy for efficiency.