Given PixiJS is a 2D renderer by default I wouldn’t expect performance as high as Three.js. Pixi3D seems to be a way to add the occasional 3D object to your 2D drawn application.

PixiJS renders 2D exactly the same as 3D is rendered using Three/Babylon, it's all WebGL (with vertex data, textures and shaders) in the end. Pixi3D should perform very similar to the other 3D renders on the web.

I expect this to have an easier story combining 3D and 2D elements than threejs, in which you are basically left alone placing html or svg elements over your canvas and synchronizing them to your 3D state (which is ok as well, you can get 2D coordinates from three for your objects with some calculating). Also three does not really support having your 3d scene with a transparent background before some other 2D or web stuff (you basically have to bring everything into three somehow, e.g. as a texture). Pixie seems to support that when they say you can have 3D and 2D Containers on top of each other.