Mac Pro Tower - Sapphire Radeon Rx580 is really, really slow I recently installed a  Sapphire Pulse Radeon Rx580 8gb on my 2010 era (with upgraded CPU) Mac Pro tower.   I did this because my former GPU, a NVIDIA GTX 970 is not "Metal Compatible" and would not allow me to upgrade to Mojave.  I selected this GPU because it was one of the few recommended GPU for Mac Pros.     I use blender a lot, and was hoping that I would get something of a boost in performance from the upgrade.

Alas, no dice.  In fact, the GPU is even slower than the CPU when rendering benchmarks like "Classroom" or "BMW".  With my GTX 970, I got a good boost, with benchmark times sometimes as much as 3x faster.   

I'm beginning to regret the "upgrade".  I don't need amazing performance, just something that makes cycles run quicker. 

Am I doing anything wrong?   Is there any patch or fix that I can download that will help? 

Thanks!
Unfortunately Apple has deprecated OpenCL and OpenGL in favor of their own Metal API. In fact, Blender has recently dropped OpenCL support on macOS completely, due to too many bugs in the macOS OpenCL compiler.

Long story short, due to apple's rejection of widely used open APIs, it's unlikely there will be any support for OpenCL rendering on macOS in the immediate foreseeable  future. Perhaps someday it will be supported via vulkan, but in the meantime it looks like you're out of luck/stuck on old versions of blender.

If you don't want to shell out the $$$ to dual-boot Windows, you might try running Linux. Historically, Linux has outperformed Windows when it comes to CPU rendering (it usually matters less for GPU rendering, but it's often a little faster there too). That said, I was unable to find any up-to-date benchmarks comparing the two, so take this with a grain of salt.