The role of time on task (TOT) for students' attainment in a low-stakes assessment of cross-curricular skills was examined using the log data collected in the computer-based assessment (CBA). Two structural equation models were compared: Model 1, in which students' test scores were explained by grade point average (GPA) together with mastery and detrimental motivational attitudes, and Model 2, in which TOT was added to the model to mediate the effects of GPA and the 2 motivational constructs. Fitting the models to nationally representative data of 4,249 Finnish 9th graders (M[subscript age] = 15.92 years) confirmed the hypothesis that investment of time plays a key role in explaining test scores in low-stakes assessment even when prior ability (GPA) is taken into account. It was also confirmed that the effects of the detrimental attitudes on students' attainment were mediated by TOT. The study makes an important contribution to research regarding the role of motivational attitudes and TOT in low-stakes assessment, which is vital for the use of the assessment results in national and international benchmarking. It is concluded that log data provide a functional way to investigate time investment in CBA as an indicator for students' effort, yielding relevant implications for educational psychologists.