It doesn't: the controller uses Wifi, not USB or Bluetooth. It communicates directly with the Stadia servers. It only has it's own internal lag + Wifi.

https://stadiafever.com/why-does-the-stadia-controller-use-w...

Note: if you want to give your Stadia controller a second life after the service shuts down, you can do that using USB. Reportedly rumble works on Steam and Xbox. Bluetooth does not work, even though there seems to be a lot of demand for that.

I've spent quite a bit on WiFi in my house and I can get almost gigabit speeds over 6e. But latency is terrible. I have no idea how they'd manage that.

If the latency is terrible, you might want to try disabling old devices or seeing if you have interference from another network - it’s normally a couple of milliseconds, maybe 4-7 if you’re using a mesh.

Latency isn’t the only issue. Bandwidth is another. MetalJesusRocks has an interesting video[0] on it.

[0] https://youtu.be/01ZTOyx-q7s

Yeah dude, if you're playing on a TV as well, one that doesn't have a way of disabling its postprocessing, you may be slapping another 250 ms on that.