Which may not be as crappy as you’d think. I really think it depends on the image size and your processing.
Exporting a 20 MPx image with my usual settings (2 instances of diffuse or sharpen, one with the preset local contrast: fine, the other with sharpen demosaicing: AA filter):
Without OpenCL: pixel pipeline processing took 14.170 secs (136.335 CPU).
With OpenCL, and darktable’s resources setting to small, and a maximum of 2888 MB GPU RAM consumption, as reported by nvidia-smi: pipeline processing took 11.255 secs (17.801 CPU).
With normal: 4594 MB GPU RAM consumed (pixel pipeline processing took 6.069 secs (10.132 CPU)).
With resources set to large, my maximum GPU memory consumption was 5.8 GB (pixel pipeline processing took 4.355 secs (5.542 CPU)), with darktable and Firefox running on KDE; the baseline was 793 MB, when everything was idle). Xorg consumed 403 MB, Firefox 217. With a lighter WM/DE, maybe Xorg’s usage could be reduced, and one can close the browser while editing.