Hi everyone,
Actually, more fuel might even make a fighter faster.
Design angle of incidence would typically be selected for top speed at full pressure altitude. Below this altitude, the fighter probably flies with a nose-down attitude, meaning that the fuselage is not optimally aligned with the airflow. More fuel and more weight means that the same lift is achieved at a higher angle of attack, which is equivalent to a less pronounced nose-down attitude of the fuselage and less parasitic drag.
If the decrease of parasitic drag is greater than the increase of induced drag, the heavier aircraft will be faster.
Regards,
Henning (HoHun)