This article identifies the profiles of students whose performance is improved by the PBL method, and the influence of certain student-dependent factors that lead to improvement. Effects on performance vary from one student to another, so we distinguish between outcomes among low-performing, average-performing, and high-performing students. To explain the differences in performance improvement among these types of students, it is necessary to analyze factors that depend on students themselves. This article proposes a mediation/moderation model that reveals the influence of prior knowledge, prior motivation, and effort on the performance improvement that PBL brings about among the three types of students. Our findings show that PBL is especially effective among low-performing students as it results in substantial improvements in their performance levels, making them less dependent on prior knowledge, prior motivation, and the amount of effort made. Performance levels do not improve among high-performing students, for whom obtaining excellent results is observed to depend more on their prior cognitive baggage.