Ok, i'm understand. Now it's works faster for smaller amount of points. Of course it's works SLOW for minimum zoom when user try to see all points. I'm also try to workaround this behaviour using QwtWeedingCurveFitter (Douglas/Peuker algorithm), but that's works slower as compared with disabled fitter. After Douglas/Peuker i see smaller amount of points. It looks like the alogrithm of QwtWeedingCurveFitter working more time than direct drawing operation of full points.