Assessing product sustainability performance is essential for supporting sustainable product development since it enables comparison and progress monitoring and informs decision-making. However, approaches for integrating sustainability considerations into product development are not merely technical choices; they reflect underlying assumptions about how sustainability performance is conceptualized and interpreted, which has significant implications for what the goal of sustainable product development is considered to be. Based on a systematic literature review, complemented by exploratory interviews with practitioners, this study examined how product sustainability performance is conceptualized in relation to what is assessed (scope), at what level impacts are considered (scale of impact), and in relation to what results are interpreted (point of reference). Three conceptualizations emerged from the clustering of the findings, showing different combinations of the spectrum of scope, scale of impact and point of reference. In addition, it was found that the choice of performance lens, i.e., efficiency or effectiveness, can be linked to the adopted point of reference. Both lenses are necessary in the product development process, and the paper therefore proposes a definition of product sustainability performance that combines both. The discussion presents theoretical implications of the proposed definition to the product development process. The paper concludes that advancing sustainable product development may depend less on creating new assessment tools and more on strategically applying existing ones within a shared conceptual understanding of product sustainability performance.