Thank you Uwe
Even I started with C/C++ few month ago I feel that I'm starting to understand the philosophy of QWT. I still have some difficulties but it needs some time to understand.
I already disabled autoscaling.
Actually I think that QWT is fast enough to fit my needs.
At least as I'm using Armadillo library I'm able to perform 2D interpolation before plotting in case when number of curves and number of of points in each curve significantly exceed heigh and width of canvas in pixels.

Thank you!