Seeing that OpenCL speeds things up even on a laptop wit only the integrated graphic card, I have now made my primitive test with a stopwatch on my ageing home PC with once gaming HD 7850 card with 2 GB of GDDR5 memory (still supported/updated by AMD). Exporting ten 24 MP cr2 files (sharpen, 2 x profiled denoise, default, colour and lightness) to jpg.
i5 3570 @ 3.40 GHz, 16 GB DDR3 RAM, 2 x HDD
win7, DT ver. 2.3.0+941~g3fe3c2749 from here
ubuntu studio 17.04, DT ver. 2.2.5 from darktable-unstable PPA
win opencl 01:31 91 s 100 %
win no opencl 04:15 255 s 280 %
lin no opencl 03:55 235 s 258 %
On Ubuntu, I wasn’t able to find a way to make OpenCL work with this GPU, and while my Linux skills are pathetic, I ran into multiple suggestions, that this may not be possible.