Purpose: In Gamma Knife radiosurgery, an efficient plan is one that achieves dosimetric quality while minimizing treatment time. Although minimization of treatment time to improve throughput and benefit patient comfort is a common and important goal of radiosurgery planning, to date no studies have attempted to specifically quantify efficiency. The aim of this study was to define simple index to score efficiency, and by quantifying time savings achieved by replanning those cases identified as least efficient, so demonstrate the efficacy of the index. Methods: To quantify efficiency, it is necessary to determine treatment times expected for specified lesions. However, because of numerous case-specifics, efficiency cannot be quantified in terms of treatment times alone. This study defines a new quantity, the attenuation-corrected normalized treatment time-dose rate product,nTRPcorr, to account for differing dose rates, prescriptions, and attenuation. A plan efficiency index (PEI) is then defined for lesions of similar volume and shape in terms of expected and planned nTRPcorr. nTRPcorr was retrospectively calculated for metastatic lesions of comparable shape. A curve fitted to data describing how nTRPcorr typically varied with volume for these lesions was then used to determine expected nTRPcorr. For each lesion, PEI was calculated as the ratio of expected-to-planned nTRPcorr. Plans with the lowest PEI were replanned, with the aim of maintaining dosimetric quality while minimizing treatment time. Dosimetric quality was defined in terms of coverage, conformity, and gradient index. Statistical significance of differences between original and replans was quantified via paired t-tests. Results: The mean(standard deviation) PEI of all reviewed lesions was 1.08(0.28). The 14 least efficient plans across the range of investigated volumes (45–19 800 mm3) had a mean PEI of 0.64, versus 1.18 when replanned (p < 0.0001). This corresponded to a mean(range) time saving of 42%(19%–62%), 29(8–52) min at date of treatment with no statistically significant (p > 0.05) change in dosimetric quality. Conclusions: The PEI is a viable metric for identifying those plans that benefit from a more efficient planning strategy.