Complex sentence: Offsetting the difference in performance was a difference in armament; the MiG-21 was armed for much of its career with the Vympel K-13 based on the AIM-9B Sidewinder.
Split into simpler sentences: Offsetting the difference in performance was a difference in armament. The MiG-21 was armed for much of its career with the Vympel K-13 based on the AIM-9B Sidewinder.