In "general" terms lower pressures will result in higher rolling resistances but it really depends on the tire design. Some tires are designed for lower pressures and actually have lower rolling resistances at these pressures. 10-15psi in 25mm tires isn't going to make that huge of a difference. There is probably some other variable or combination of variables affecting your performance. I would guess that it's probably the temps that are causing your drop. It takes longer to warm up and you generally ride slower when it is really cold and/or really hot.