The Job Demands-Resources Theory is often used as a model that explains how work conditions (job demands and job resources) have a negative or positive impact on employees' well-being and organizational outcomes. Over the years, the theory has been expanded by introducing personal resources that mitigate the negative effects of job demands, and the latest development in the theory adds personal demands as a determinant of personal and work outcomes. Therefore, the aim of this research was to examine the relationship between maladaptive perfectionism, external locus of control, and workaholism with burnout and work performance and to test whether burnout mediates the relationship between personal demands and job performance. Job performance was operationalized as task performance and organizational citizenship behavior. We focused on a specific population of healthcare professionals who often face high work-related stress, emergency situations, and emotional challenges when dealing with patients. We collected data through an online survey from 160 healthcare professionals in the Republic of Croatia. The results showed that higher workaholism and a higher external locus of control predicted burnout, while higher workaholism and lower external locus of control predicted organizational citizenship behavior of healthcare professionals. Therefore, external locus of control and, to some extent, workaholism, have been identified as personal demands in the sample of healthcare professionals. Maladaptive perfectionism did not emerge as a personal demand in this sample. Additionally, none of the variables predicted task performance, and burnout did not mediate the relationship between personal demands and either task performance or organizational citizenship behavior. The paper also discusses some practical implications of the results.