**iMac Pro 8 Core
Mac OSX 10.14.3
Vega 64 16GB
Blender 2.8 2019-13-06 03:53**
When using the iMac Pro with an external monitor which is 1080p -
Full screen and windowed mode on just the iMac everything qworks fine and cycles updates fast and responsive.
When using dual monitor ( 3D viewport + toolbars on iMac and Node editor/ UV maps on external display) very slow response time with updating colours/textures with the node editor.
Object will turn grey and blender with show that it is 'initialising' and no samples have started.
Start Blender as usual, change to cycles render and apply any form of colour shader node to the default cube.
Turn on render preview.
In single screen mode (all windows on iMac Pro) Change the colour of the shader node to any other colour.
This should result in the object changing with no delays.
Create a new window with the shader editor as the main view and move onto the second display. (unaware if resolution will make a difference but my display is 1080p)
Now when changing the colour on the shader, the default cube will change to grey and I will have either a very long delay or no change from there at all.
Moving (or closing) the second window from the external display to the iMac will remove the issue in question,
Please bear in mind, this seems to happen on a much more extreme level, the more detailed the shader is. But on the default cube with a default principled shader it will still show the same issue.
Blender 2.79's latest build does not behave the same way. It works fine on both the iMac single display or using dual display.
Hope this makes sense. Please feel free to ask for any more information, or video examples.