I think it is using OpenGL. I recall I tried Metal and it seemed to break some shaders.
It has occured to me that the 'standard' setting on the opening resolution dialog thing might not actually do anything! :o Ideally I will aim to get rid of this dialog and have resolution switching in-game too.
The quality settings are actually handled in game. I would be interesed to know if the settings on the in-game menu change the performance when switching between high and low? (the menu is the top right button).