One essential question regarding Maximum Power Point Trackers (MPPTs) is how fast an MPP tracker must be so that no unduly high energy losses occur. This question has been discussed controversially in the past, and many of the statements have not been based on real measurements, but manly on assumptions and wishful thinking of the companies' advertising department. In this article, various influences on the position of the MPP and their typical time constants will be presented. These are compared to the reaction time or speed of typical MPP search procedures. In particular, measurement results from a real solar generator will show that the partial shading caused by rapidly passing clouds and the associated significant short-term changes in the MPP voltage are not relevant to the energy output of the whole system.