I have a rock solid 100k Internet Connection at Home.
But while playing stadia on chromecast I get some weird compression artifacts. This does not happen while playing on pc or laptop via chrome
Any warning messages about the connection quality popping up when this happens? You can press the Stadia button on the controller and manually check what the reported connection quality is as well to rule this out.
If you're on WiFi it may not matter how fast your internet speed is if the location of the CCU is in gets a poor wireless connection or encounters a lot of interference.