I saw this post and did a couple quick tests. When I enable open CL in the System Resources preferences, it’s either has no effect, or is much slower. My CPU is a modern AMD 8 core Ryzen 7 CPU. Should be pretty fast.
I have an Nvidia graphics card that’s a couple years old. I don’t remember if I’m using the drivers shipped with Kubuntu 17.10, or if I installed updated drivers from Nvidia.
I took a photo I have at 6000x4000 pixels in 32 bit gamma floating point mode. I changed the B curve in the colors → Curves dialog. It took about 9 seconds for the change to reach the bottom of the monitor, and another 9 seconds when I clicked on “reset” in the curves dialog. This seems slow to me. No performance difference regardless of whether or not I had open CL enabled.
I did a similar test with Unsharp Mask, since I know that’s a GEGL operation. With Open CL disabled, it took between 9-12 seconds to get through the whole image. With OpenCL enabled, it took between 11-15 seconds, so it seems that based on this test, OpenCL slows me down.
What can I conclude from this, or what can I do to figure out if there’s a way to boost GIMP performance? Gimp 9.8.X was much faster. Maybe because it was 8-bit, so easier for the processor to do the math?