The then-new Spitfire Mk I in mid-1938 is shown in the picture.

It is equipped with a fixed-pitch, two-blade wooden propeller, and a flat-top canopy (the bulged canopy appeared soon).

The wooden propeller limited the performance of the aircraft and it was replaced by a much heavier two-pitch, three-blade metal propeller.
The new propeller increased the max speed from 361 to 365 mph and reduced the TO roll from 420 to 320 yards. The rate of climb should have improved as well.

A captured Spitfire Mk I with a two-speed propeller was captured by the Germans and compared against the Bf 109E-4 by Werner Moelders in the Rechlin test center just before the Battle of Britain.

His conclusion was that despite the Spitfire's performance (which was similar to the Bf 109E) and maneuverability it was inferior to the German fighter.
He disliked the Spitifire's inability to blunt dive without the engine losing power and the fixed pitch propeller which rendered it inferior in the climb.

By the time the Batte of Britain started most (if not all) of the Spitfires were equipped with an even heavier constant-speed propeller. This eliminated the drawbacks in the climb, although the problem with the Merlin remained.

The French also captured a Bf 109E-3 and they and the British did thorough testing. They were very dismissive and concluded that the Hurricane I and the Spitfire I were clearly superior to the German fighter.
It was typical of both sides to conclude that their fighters were better than the enemy's.
This was in part because each side had different tactics and the fighters of their side were fine-tuned to such tactics.
The other reason is that the differences in performance were not that significant.