Which version are you using ? Use 2.5.1, we changed so behavior so the DMX output works even if there is no OpenGL view visible. There might be an issue with Mac OSX which would not use the graphics card but do the OpenGL on the CPU (with a software OpenGL renderer), so it might be a bit slow depending on how many fixtures you are using, the type of medias etc.
The best is to plug a display simulator on your computer, like this: https://www.amazon.com/CompuLab-fit-Hea ... B00FLZXGJ6
With this Mac OSX will think there is a screen connected and it will use the graphics card normally