Hi, I’m running the latest manjaro version, and darktable is unable to use opencl and is painstakingly slow. I’ve been using self-built versions of darktable on windows, and opencl worked there.
I was hoping that switching to linux would add a little performance-boost, but without opencl it’s way worse instead 
I would really like to stay with linux, but I’m at a loss as to where to start troubleshopoting this. I’ve already installed all opencl packages relevant to my system that I could find through the package manager.
Which graphics card are you using? What is the output of clinfo? If by chance you are running an AMD card you need opencl-amd from AUR as mesa does not support the image extension yet and thus dt will not see opencl support
If you run darktable from the command-line with -d opencl -d perf, what do you see? And yes, more details (like the card and the driver, as @piratenpanda mentioned) would be useful to know.
You may find the following topics relevant:
https://discuss.pixls.us/t/opencl-analysis-darktable-much-faster-with-opencl-disabled-something-wrong/