My hardware is imx6 quad running Windows CE7 = WEC7 = Windows Embedded Compact 7.
I turn on the GPU and I see performance gains for small rectangles up to 500 x 900.
For larger rectangles, there is no performance improvement. See graph.
This appears to be a bug since many displays have much higher resolutions.
It seems the GPU software stack should, if necessary, break the problem down into chunks if 500 x 1000 is too big.