Hard to say.
The CPU isn't the best. Jaguar core, low clock speed, high core count.(most emulators to my knowledge thrives best with the opposite) We know the GPU is good for 6 teraflops at 32FP compute. Puts it at around a Nvidia GTX1070 (about 6,5TF)
When you're in Dev mode you get access to a performance monitor. The CPU usually hovers around 15% while emulating PS1(So not completely useless
). The GPU graph barely moves. So all that can be said, there is plenty of reservers for more demanding tasks
Raw performance, the base Xbox One should outperform Nvidia Shield by a wide margin. If we are ignoring software support.
Biggest hurdle is probably the lack of OpenGL support. So for a core to work, it also needs to support DirectX. And be somewhat optimized for it.
I'd be happy if someone got the Dolphin core up and running. As of now, the newest console that has a working core is PS1. And the Xbox One X runs that flawlessly. While my Switch, fully clocked to docked speeds struggles with some of the shaders, Xbox One X takes pretty much any shaders you throw at it.
EDIT: PS1 isn't really the newest one available. That would be PPSSPP, PSP emulator. But that is not a part of retroarch (the retroarch core does not support DirectX)