• Unspeeded response time (URT) is negatively correlated with absolute valence and arousal. • URT is larger for food images with ambiguous valence. • URT is shorter for food images eliciting more arousing or intense affective responses. • The URT may be an implicit evaluation measure that complements self-report measures. Affective responses are often adopted as proxy measures of potential food choices. To reliably assess affective responses there is a need for implicit measures that are less prone to cognitive biases, context, lack of introspective capacity, social desirability, and intercultural differences than the explicit self-report measures that are commonly used. In this study, we investigated the relation between unspeeded response time (URT) and the affective appraisal (in terms of valence and arousal) for food images. We find that URT is negatively correlated with both absolute valence and arousal: URT is larger for food images that are rated near-neutral (ambiguous) on valence and low on arousal than for images eliciting more extreme positive and negative affective ratings. Participants need more time for the affective evaluation of food images with lower emotional clarity than those with clear-cut emotional quality. Hence, the URT may serve as a continuous and easily observable implicit evaluation measure that complements self-report measures.