my workflow in Darktable is that I have it open on my smaller monitor and use the external preview on my big color profiled 4k screen in full. Unfortunately while the preview in the main window is updating in real time when I change sliders, the external preview is only updating when I let go of the sliders and a few seconds after.
I would like to either disable the internal preview and only use the external one but as fast as the internal, or swap the behavior of both around so that the external preview updates fast and the internal one slow.
Is that at all possible? If not, might this be a desirable feature for other people too?
That in itself explains a lot about the difference in update speed:
if your big monitor is twice the size of the small one, there are already 4× as many pixels to calculate. That alone requires much more CPU/GPU time. In addition, the large preview may have become too large to fit in processing memory, meaning that the image has to be calculated in tiles. Those tiles need overlap in many cases (how much overlap is required depends on the module and its parameters), which means even more CPU/GPU time…
I agree with Remco’s comment. High resolution displays require more processing (preferably GPU) power. Furthermore from own experience the colors and tones are less reliable on the 2ndary preview. I understood that performance has priority over image quality there.
The only way to make it faster is to make your computer (GPU in particular) faster.
To understand why, you’ll have to understand how darktable works. Internally each preview has its own processing pipeline, that works on the image at the resolution it’s being shown. So with one 1080p screen, for example, there will be two internal pipelines, one low-res for the navigator in the upper left corner and one for the main preview, which will be somewhat less than 1080p. In your case there are three pipelines, one of which will be working with almost 4k of data. Unless you have a very beefy GPU, working on a 4k screen is going to be slow even on it’s own, but you then have another screen that also needs to be updated on top of that.