This paper compares the factors affecting completion times (CTs) to web survey questions when they are answered using two different devices: personal computers (PCs) and smartphones. Several studies have reported longer CTs when respondents use smartphones than PCs. This is a concern to survey researchers because longer CTs may increase respondent burden and the risk of breakoff. However, few studies have analyzed the specific reasons for the time difference. In this analysis we analyzed timing data from 836 respondents who completed the same web survey twice, once using a smartphone and once using PC, as part of a randomized crossover experiment in the LISS panel (Longitudinal Internet Studies for the Social Sciences). The survey contained a mix of questions (single-choice, numeric entry, text entry) that were displayed on separate pages. We included both page-level and respondent-level factors that may have contributed to the time difference between devices in cross-classified multilevel models. We found that respondents took about 1.4 times longer when using smartphones than PCs. This difference was larger when a page had more than one question or required text entry. The difference was also larger among respondents who had relatively low levels of familiarity and experience using smartphones. Respondent multitasking was associated with slower CTs, regardless of the device used. Practical implications and avenues for future research are discussed.