turn out I had to manually install from manjaro repo the opencl-mesa package… now darktable is much much faster! Thanks to a stranger on the internet replying to a youtube comment I left saying how much slower darktable is that lightroom. Thank you kind stranger. Video here: Adobe Lightroom vs Darktable - Battle of the RAW Editors - YouTube
Heh just noticed youtube still has my real name… I guess left over from G+ as I wouldnt have changed it after that. Meh whatever.
sooo… this took a LOT more work than just installing opencl-amd from the manjaro repo. I thought I had enabled this but turns out I had not! Placebo, much?
I had to get the AUR one in the end as darktable would not recognise the 5700XT as a valid openCL device, and failed to enable openCL on startup.
I ran darktable -d opencl to see what it was doing.
After a LOT of searching online as there’s not a lot of info on doing this in Manjaro/Arch, I installed the AUR version of opencl-amd and then re-ran darktable… and it’s enabled yay!
Much faster. 
Thank you for sharing the progress, i’m glad you got it working. These hardware issues can be tricky.
Ok so maybe hold that thought - did an update and rebooted and now xorg cant see any display/screen device 
Luckily I dual boot, so can revert to the old WIn10 disk; and the files I need I have on a shared ntfs drive so I can carry on working until I work out WTF I screwed up.
update: I may have added /etc/X11/xorg.conf.d/20-radeon.conf - which I removed and it booted… damn you Arch Wiki and all your options on how to do things. 
So yeah, opencl wil be fine, and I am still an idiot 
That is how I have had to get OpenCL working too. I wish RawTherapee could use it as well.
Can confirm aur/opencl-amd is needed for dt to access GPU.
