Hi! Just want to share this that i recently had lagg in shadow of Tomb raider many times and it suffered too often. After restarting the router/modem it runs perfect without no lagg at all, before this it just got lagg for 2-3 seconds every few minutes. So try this out if you have problems.
Also if you have issues and you're using Google WiFi there is an option that you can turn on in the Google home app to give Stadia preferential network treatment. It definitely helps.