это код из Qwt, который все проясняет.

static inline void drawPolyline(QPainter *painter,
    const QPointF *points, int pointCount, bool polylineSplitting)
{
    bool doSplit = false;
    if ( polylineSplitting )
    {
        const QPaintEngine *pe = painter->paintEngine();
        if ( pe && pe->type() == QPaintEngine::Raster )
        {
            /*
                The raster paint engine seems to use some algo with O(n*n).
                ( Qt 4.3 is better than Qt 4.2, but remains unacceptable)
                To work around this problem, we have to split the polygon into
                smaller pieces.
             */
            doSplit = true;
        }
    }

    if ( doSplit )
    {
        const int splitSize = 20;
        for ( int i = 0; i < pointCount; i += splitSize )
        {
            const int n = qMin(splitSize + 1, pointCount - i);
            painter->drawPolyline(points + i, n);
        }
    }
    else
        painter->drawPolyline(points, pointCount);
}