For multinational corporations, internationalization is a core strategy for corporate growth and performance. Growth through internationalization brings economies of scale and scope, as well as various forms of economies of growth. On the other hand, as the level of internationalization increases, so does the level of complexity that firms experience. Therefore, multinational corporations often find themselves in strategic dilemmas related to their level of internationalization. Although numerous attempts have been made to empirically verify the effect of internationalization level on firm performance, the research findings remain inconsistent. This may be because the relationship between internationalization level and firm performance is far more complex and dynamic than researchers have previously suggested, or because researchers have focused solely on the level of internationalization while overlooking the process of internationalization through which that level was achieved. In other words, even multinational corporations exhibiting the same level of internationalization may achieve different performance outcomes if they went through different processes in executing their internationalization strategies. This study approached the internationalization process from three dimensions—speed, scope, and regularity—and accordingly proposed a total of four hypotheses regarding the relationship between internationalization level and firm performance, as well as the moderating effects of the three process variables on this relationship. These hypotheses were tested using a research sample consisting of 218 manufacturing firms listed on the Korea Stock Exchange as of March 2009. Regression analysis revealed a U-shaped relationship between internationalization level and firm performance. However, this relationship may vary depending on the internationalization process. First, the speed of internationalization was found to have a positive moderating effect on the relationship between internationalization level and firm performance. That is, the faster the speed of internationalization, the more firm performance deteriorates before reaching the optimal level of internationalization, but improves more sharply beyond the inflection point. Meanwhile, the regularity of internationalization also showed a moderating effect: the more irregular the internationalization process, the more the relationship between internationalization level and firm performance shifts from a U-shape to an inverted U-shape.