Mac Pro Tower - Sapphire Radeon Rx580 is really, really slow

Or you can use VirtualBox to Virtualize a Linux System ( eg. Ubuntu Studio ) and use PCI-Passthrough to give it direct Hardware access to the Video card. If your Mac has proper iommu Support that should be the best Option without dualbooting.

Check:

The intel_iommu=on boot option could be needed. Search for DMAR and PCI-DMA in kernel boot log.

Once you made sure that the host kernel supports the IOMMU, the next step is to select the PCI card and attach it to the guest. To figure out the list of available PCI devices, use the lspci command. The output will look as follows:

01:00.0 VGA compatible controller: ATI Technologies Inc Cedar PRO [Radeon HD 5450] 01:00.1 Audio device: ATI Technologies Inc Manhattan HDMI Audio [Mobility Radeon HD 5000 Series] 02:00.0 Ethernet controller: Realtek Semiconductor Co., Ltd. RTL8111/8168B PCI Express Gigabit Ethernet controller (rev 03) 03:00.0 SATA controller: JMicron Technology Corp. JMB362/JMB363 Serial ATA Controller (rev 03) 03:00.1 IDE interface: JMicron Technology Corp. JMB362/JMB363 Serial ATA Controller (rev 03) 06:00.0 VGA compatible controller: nVidia Corporation G86 [GeForce 8500 GT] (rev a1) The first column is a PCI address, in the format bus:device.function. This address could be used to identify the device for further operations. For example, to attach a PCI network controller on the system listed above to the second PCI bus in the guest, as device 5, function 0, use the following command:

VBoxManage modifyvm "VM name" --pciattach 02:00.0@01:05.0 To detach the same device, use:

VBoxManage modifyvm "VM name" --pcidetach 02:00.0

..

As a workaround, you can perhaps invest a bit in a skeleton Windows PC with a high end graphics-card and use it as a render client.

Here's an awesome tutorial for that from CG-Cookie. https://www.youtube.com/watch?v=FNhUnPWzVaw

If you want to do GPU rendering, your only option is to not use Cycles and use a Metal supported renderer (Radeon ProRender) instead:

Radeon ProRender

As previously mentioned, OpenCL rendering is no longer supported in Blender on Macs.

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.

Yes, you doing it all wrong. Macmetal support of opencl gpu rendering in cycles is non-existant.

https://linustechtips.com/main/topic/975081-cuda-1060-6gb-opencl-rx-580-for-blender/

Just install Windows 7-10 on mac as second os. Thats the optimal solution I think, and easiest one. My rx580 is really much faster (like 10-20 times faster) in cycles rendering than cpu.