I just found something strange by accident: it looks like setting opaque to true does increase GPU usage.
Removing all setOpaque calls in different components of the plugin I am working on, GPU usage is now similar to that of JUCE 7 D2D (software renderer is still much slower though).
This can be verified with the example code I posted above. calling setOpaque(true) for TestAnimation, and of course calling fillAll in paint(), does increase GPU usage by a substantial factor:
- setOpaque(false), animation on the same size: 35%
- setOpaque(false), animation on opposite side: 45%
- setOpaque(true), animation on the same size: 55%
- setOpaque(true), animation on opposite side: 75%
(14900k iGPU, 4k 60Hz display, maximized window, release build)
Edit: strangely enough, I see the same phenomenon in JUCE 7 D2D, and GPU usage is even greater than JUCE 8 in the above example, but for some reason this is a different story in my plugin. Tracking down the exact thing that makes JUCE 8 slower in my app might be difficult. In any case, dropping setOpaque does close the gap for me, at least for the d2d renderer.