There’s not a whole lot going on in frame that will impact framerate to that extent, it would mostly be what assets you use which still shouldn’t get that crazy. Things that typically affect rendering time like lighting and tracing reflections aren’t an issue here since it’s designed for real-time performance. Even at a crawl it will be faster than you would typically render something. Archviz renders can often take an hour to render an image, here you can do it in seconds. Even if it takes 10 seconds vs. 1 second it is still way faster.
Also, they didn’t use UE4 to render Rogue One, so that’s irrelevant.