Increased heart rate directly increases cardiac output because cardiac output is the product of heart rate and stroke volume (CO = HR × SV). Raising the heart rate increases the number of cardiac cycles per minute, thus increasing the overall volume of blood pumped by the heart. This is a primary mechanism by which the body adapts to increased metabolic demands. In contrast, decreased preload, decreased contractility, or increased afterload generally reduce stroke volume and therefore do not directly enhance cardiac output. This makes increased heart rate the most direct way to boost cardiac output acutely.

Reference: Guyton and Hall, Textbook of Medical Physiology, 13th Edition.