Hey, I have the same issue, with latest (4.21) UE.
Blank project, import 2 textures, create material from (diff, png) texture, add emissive (png), save, and upon save (or apply) I get the same “Apply changes to the original material and its use in the world” message and the whole UE freezes, CPU usage is over 50%, but doesn’t appear to do anything. I’m following a tutorial video and this should be a 1 sec stuff, even in my older machine it shouldn’t take more than a few. tried restarting a few times, same result.
After the 1st texture, I left it there for 30+min, I created another one and there only seen the message for about 3 seconds.