Hey @Field,
have you tried already to reboot the chromecast or try to use a different ethernet cable and check if everything is plugged in correctly? These steps help normally, but you can also contact the Stadia support as they might have more troubleshooting steps to help you or they can even help you if your chromecast is defective.
Hope the best for you!
According to RTINGS your TV model should have great input lag but only in game mode. You can see outside of game mode it's pretty bad and over 100 ms. Make sure that Automatically enable game mode is set to ON in your CCU settings via the Home app.
Also for your TV model you have to enable Instant Game Response for the HDMI port in use in your TV settings.