To clarify, Darktable is taking half a second even with just the base curve active. Exposure adjustments are never real-time (at least on my computers, but they are fairly recent and fast).
On further inspection, it seems that Capture One is only rendering a downscaled version of the image, or if zoomed in, only a portion of it. The former is obviously visible as lack of sharpness, the latter as panning intermittendly exposes un-rendered areas.
Does Darktable do something similar? Are earlier stages in the processing pipeline cached when modifying a later stage?