What exactly is the state of Linux GPU support. For ex, Unreal users report lower frame rates than on Windows. So the problem could be with Epic GLSL code ( they do mention that their code was automatically generated from their HLSL code, so it shouldn't be to efficient ), the OpenGL library and NVIDIA drivers. Where exactly do you think is the problem here?
Also, I want to do GPGPU in OpenCL but Vulkan mentions that it's also for computing. Should I understand that Vulkan superseded both OpenGL and OpenCL?