Computer-based assessment can provide new insights into behavioral processes of task completion that cannot be uncovered by paper-based instruments. Time presents a major characteristic of the task completion process. Psychologically, time on task has 2 different interpretations, suggesting opposing associations with task outcome: Spending more time may be positively related to the outcome as the task is completed more carefully. However, the relation may be negative if working more fluently, and thus faster, reflects higher skill level. Using a dual processing theory framework, the present study argues that the validity of each assumption is dependent on the relative degree of controlled versus routine cognitive processing required by a task, as well as a person's acquired skill. A total of 1,020 persons ages 16 to 65 years participated in the German field test of the Programme for the International Assessment of Adult Competencies. Test takers completed computer-based reading and problem solving tasks. As revealed by linear mixed models, in problem solving, which required controlled processing, the time on task effect was positive and increased with task difficulty. In reading tasks, which required more routine processing, the time on task effect was negative and the more negative, the easier a task was. In problem solving, the positive time on task effect decreased with increasing skill level. In reading, the negative time on task effect increased with increasing skill level. These heterogeneous effects suggest that time on task has no uniform interpretation but is a function of task difficulty and individual skill.