Hiho,
As heart frequency increases, the Bowditch effect allows for increased contractility and thus increased cardiac output;
Meanwhile, the diastole is the phase shortened the most and therefore there is a lessened ventricular filling time with decreased ventricular filling per cycle. What should follow is a decrease in ventricular preload which will in turn lower contractility (Starling).
Empirically, we see our heart frequency increase as cardiac stress (i.e. exercise) increases, so obviously the first "overrules" the latter, but I wonder if anybody can explain the why and how of this phenomenon?
Rik