This version was capable of a maximum speed of 342 mph , some 25 to 30 mph faster than the Mk I. This version was capable of a maximum speed of 342 mph , some 25 to 30 mph faster than the Mk I. The performance of the Spitfire over Dunkirk came as a surprise to the Jagdwaffe , although the German pilots retained a strong belief that the 109 was the superior fighter .