I think I'm talking about something different.
My plotted curves have some specific features. They are like deviation from some vertical baseline (and each curve has a baseline).
Lets suppose that canvas width is 500 pixels. Every curve has QPen width equal to 1 and if all curves are settled equidistantly on the canvas then there is no much sense to plot more than 500 my "pseudovertical" curves.
So if I try to plot 5000 curves then I can choose every tenth (0, 10, 20, ...) curve and then plot it.

And the same for Y axis. Every point of every of my curves is settled equidistantly (Y axis of every point is something like : 0, 2, 4, 6, 8 ...). So linear interpolation is fast and could improve plot perfomance when number of points much bigger than canvas height