Building on earlier work that explored within-student variation in hours of instruction across school subjects, we investigate the impact of instruction time on student test scores in Switzerland, as measured by the PISA 2009 test. Our results confirm the results of previous studies of a positive effect of instruction time on student performance. Moreover, we find considerable heterogeneity in the effectiveness of instructional time across ability-related tracks, with the more able students benefitting more. Additional instruction time increases the within-school variance of subject-specific test scores, indicating an increase in educational inequality.