Many low-stakes assessments, such as international large-scale surveys, are administered during time-limited testing sessions and some test-takers are not able to endorse the last items of the test, resulting in not-reached (NR) items. However, because the test has no consequence for the respondents, these NR items can also stem from quitting the test. This article, by means of mixture modeling, investigates heterogeneity in the onset of NR items in reading in PISA 2015. Test-taking behavior, assessed by the response times on the first items of the test, and the risk of NR item onset are modeled simultaneously in a 3-class model that distinguishes rapid, slow and typical respondents. Results suggest that NR items can come from a lack of time or from disengaged behaviors and that the relationship between the number of NR items and ability estimate can be affected by these non-effortful NR responses.