CPU usage due to curves with many points

[Uwe]

Now, since the CPU usage is apparently due to continue replots, could the use of QwtPlotDirectPainter::drawSeries make any difference?

Sure - you draw the new points only on top without repainting the previous parts.
You could also have a look at a development branch( >= 6.2 ). For certain situations the new QwtPlotCurve::FilterPointsAggressive flag can make the difference.

Uwe