I am using QwtPlotSpectrogram with contour mode on and QwtPicker in a Plot. I noticed that when I have my contour ON my picker (showing tracker text) is slow compare to contour mode OFF. It seems like "drawing the contour lines" is called many times even my data has not changed at all. I believe QwtPicker is using some sort of qwtoverlay widget to minimize redrawing.

Now, Did I miss any cache setting for Contour mode? How can I make my picker as responsive as contour mode OFF in my spectrogram?