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OpenCL TAPI mixed performance

I'm getting mixed results when using the OpenCL transparent API in terms of performance, so I wrote a simple test application for measuring the execution time of a few OpenCV methods. I'm testing the methods Sobel, mean and sum with 5000x5000 matrices on CPU and GPU. The methods are called 10 times, with an additional call before starting the measurement as advised in another Q&A post. The code is found here

The following results were obtained on the same machine, using a GTX 980 GPU with the latest drivers on both Linux and Windows, built with OpenCV 3.2:

OpenCL devices:
GeForce GTX 980

               Ubuntu     Windows
sobel on cpu:  224ms      234ms
sobel on gpu:  44ms       593ms
mean on cpu:   85ms       78ms
mean on gpu:   399ms      500ms
sum on cpu:    86ms       78ms
sum on gpu:    7ms        15ms

The CPU results are perfectly comparable. On Linux the GPU results from Sobel and sum are reasonable, I guess. The speedup is 5x to 10x. On Windows however the Sobel performance is very poor and sum is two times slower. What could be the reason for this?

Also note that on both systems the performance of mean is very poor, which is why I'm currently using sum as a workaround for now. Is this because the OpenCV functions must be explicitly optimized for OpenCL and mean just didn't get the attention yet? If so, is the current state of OpenCV support documented somewhere or do we have to write tests like this?