Surveys completed on mobile web devices (smartphones) have been found to take longer than surveys completed on a PC. This has been found both in surveys where respondents can choose which device they use and in surveys where respondents are randomly assigned to devices. A number of potential explanations have been offered for these findings, including (1) slower transmission over cellular or Wi-Fi networks, (2) the difficulty of reading questions and selecting responses on a small device, and (3) the increased mobility of mobile web users who have more distractions while answering web surveys. In a secondary analysis of student surveys, we find that only about one-fifth of the time difference can be accounted for by transmission time (between-page time) with the balance being within-page time differences. Using multilevel models, we explore possible page-level (question-level) and respondent-level factors that may contribute to the time difference. We find that much of the time difference can be accounted for by the additional scrolling required on mobile devices, especially for grid questions.