We have the same issue too. I have a 3080 and we have a master materials with many (1000s) of instances from it. We are not using Nanite or Lumen or anything like that. It does usually save eventually but can take maybe 40 minutes or so. I am wondering if it’s recompiling all the shaders but there’s no feedback about what it’s doing. Right now I am trying to save a new material instance, texture and simple model. Sometimes it’s jsut fine saving and other times it stalls like this.