I would bet that it is 90% psychological and a bit of physics. When I ride a bike that is really stiff - I feel fast. There is no way I can truly be so much faster that I can feel it. If you weighed the shoes before you rode and disparaged about the weight, then most certainly anything wrong with that ride will be due to the weight. Ultimately, if you are not racing does it matter? Just reset your baseline and enjoy new shoes. If you bought them to be faster than I would just ask why does faster matter?