Spitfire Mk.I strange behaviour

I have to test it myself.

One thing I like to mention that should be considered when interpreting the results:

The weight in itself has no direct influence on the speed as such. The only impact on speed is due to induced drag as more angle of attack has to be applied for a higher weight which will increase the drag from which a reduced max speed results. This may be not extensive so only a little speed loss may be all right. The weight will overall be however very disadvantageous for acceleration.

However it shoulf of course not go faster with higher mass.