That is not exactly correct. OpenGL is always used to display the video, and it often does a final step of colorspace conversion and scaling to fit the window. If Display Method = DirectX, then OpenGL is converted to DirectX. Here the GPU is used for preview but not for the image processing in filters, transitions, and track blending. You can turn on that unstable feature in the configuration file. However, it will not accelerate existing projects made using the CPU effects. I will not tell you further how to find the config file and turn it on because it is unsupported and this is for advanced users willing to make tests to see if it works OK for them. An advanced user will know how to locate the configuration document on the web site. Do not report bugs while this option is enabled.