Previous studies on information overload have failed to present consistent results. This study, viewing information overload not as a general phenomenon but as one influenced by other variables, analyzed the effect of information quantity on decision-making performance by considering the level of involvement and time pressure. Recognizing measurement problems in assessing decision-making performance, this study employed not only the optimality criterion-based measurement method used in prior research but also methods utilizing Euclidean distance and satisfaction range. The empirical analysis of the information overload hypothesis, taking into account the mediating effects of the level of involvement and time pressure, revealed that information quantity, level of involvement, and time pressure all affect decision-making performance. In particular, the information overload phenomenon was found to occur when time pressure exists in decision-making.