Thank you James,
Yes, I was referring to Metal compute acceleration, apologies if that wasn't clear. The check box label in the preferences pane says "Hardware Acceleration:" and beyond the check box it further says "Enable Metal compute acceleration". Below that in brackets then it says (No compatible GPU).
If I understand you correctly, this option is specific to the Intel CPU's onboard graphics processor and not the discrete GPU then?
I have a Macbook 12" here which is upgraded to High Sierra so I downloaded the 1.6.6 version of Affinity to test it.
The Macbook has no dedicated GPU only the intel HD 515 that is integrated on the CPU. Now with this configuration the Hardware Acceleration: (Metal compute acceleration) is available and in testing the lighting filter on an image it is MUCH faster and smoother than the imac. This obviously should not be the case since the imac has a way more powerful dedicated graphics processor than the Macbook 12.
So perhaps High Sierra is the difference here and is needed to enable this option, and this option will provide performance improvement on dedicated graphics processors like I have in the imac also?