It is not really important whether CPU or GPU are used. Using both a little does not mean that the responsiveness gets better. It is all about the preview rendering architecture of the tool. How the data to process is calculated on the visible part of the image and the current zoom factor. How the visible part is divided in computational chunks. How the computational chunks are distributed to cores/shader units. Which correction algorithms are used, whether they support parallel processing, the order of them in the processing pipeline and so on. I have adressed this topic more general in: Avoid Blurry Previews
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