After I upgraded to DT 3.0x I have been missing Opencl. Very annoying and frustrating as things takes 5-8 times as long, especially importing new images and exporting series requires coffee without Opencl, and adjusting a view takes a little.
But luckily I have now found the cure, my AMD R285 GPU needs a little more than standard AMD GPU driver install on Ubuntu (18.04 or 20.04 don't matter). The procedure - requires a little technical courage - can be found here:
Thanks for the tip Niels, it will be helpful to others for sure