First of all I would optimize the existing solution and turn all paint caches off, because you have your own image cache:

a) Spectrogram item
Qt Code: 
  1. spectrogram->setCachePolicy(QwtPlotRasterItem::NoCache);
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b) Plot canvas

Qt Code: 
  1. plot->canvas()->setPaintAttribute(QwtPlotCanvas::PaintCached, false);
  2. plot->canvas()->setPaintAttribute(QwtPlotCanvas::PaintPacked, false);
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c) Qt ( only supported on X11 )

Qt Code: 
  1. plot->canvas()->setAttribute(Qt::WA_PaintOnScreen, true);
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Another optimization on multicore systems is to render parts of the image parallel in several threads.

Uwe