You can try rendering a 2D rectangle as 2 quads in whatever GPU API you like the most, then try pushing the same amount of pixels to a CPU buffer via the widest CPU memory write instructions you can have (and then wait the GPU-accelerated compositor upload them to the GPU anyways to present them...).
I can tell you for a fact one will be 2 orders of magnitude faster, want to make bet which one?
Using a GPU api does not imply you have to render at the screen's refresh rate, or even that you have to update the entire window.
You can try rendering a 2D rectangle as 2 quads in whatever GPU API you like the most, then try pushing the same amount of pixels to a CPU buffer via the widest CPU memory write instructions you can have (and then wait the GPU-accelerated compositor upload them to the GPU anyways to present them...).
I can tell you for a fact one will be 2 orders of magnitude faster, want to make bet which one?
Using a GPU api does not imply you have to render at the screen's refresh rate, or even that you have to update the entire window.