The buffer is about lengthXheightXsizeof(double)=1024x200x8=1.6MB big. Is this too much data?
For the image composition the size/resolution of the target paint device ( in your case the size of the plot canvas ) is a maximum. Guess in your case the resolution of your data is way below that and you should implement the QwtRasterData::pixelHint() method.
Considering a canvas of 1200x1200 pixels the loop ( calling value() in each iteration ) would be reduced from 1444000 to 204800 for your data. How much effect you will have of course depends on the geometry of your canvas, but in this example only 15% of the loop would be left ( In case of using QwtMatrixRasterData the pixelHint method is provided automatically ).

Is there a way i can shift the current waterfall by one pixel and only draw the last line?
Sure by overloading renderImage(). But I don't have the details before having tried this myself - that's why I will implement such an example some day.

Uwe