Thank you for your quick response Uwe!
It helped me to solve the problem with the performance.

I have a question about displaying (plotting) data. Resolution of the data is much higher then the resolution of the screen.
Usually the date consists of a lot of low values (5 - 10) and rare single high values (90 - 100). When we display data on spectrogram we often do not see this peak (high value).

Is it possible to do not lose these single maximums on the spectrogram?

I am looking for an analouge for QwtPlotSpectrogram for method "setPoint Attribute (QwtPlotCurve :: FilterPoints, false)" that used for QwtPlotCurve to don't loose peaks?