How to make the number of FPS (frames per second) depend on the calculation time in the For cycle and was less than or equal to 60?

Qt 4.8.4
Why is FPS changing and not stable? How to make FPS to be as high as possible and lower than 60 and if the calculation time for For changes?
I draw a circle of i <10,000 points. Operating time For 0.01 sec.
I will change the parameter and draw from i <1 000 000 points, then Work time For 0.95 sec.
How to make FPS decrease depending on the complexity of the calculations from the time of the For loop and were at least approximately constant? Now they change much and are not stable. Incorrect slider update frequency?