The missions will make sense without Return to the City, so don't sweat it. I am sorry for its performance-related problems, but it was built with methods that work in Dromed and don't work in DarkRadiant. (In short, you have to watch for polygon count in your direct view in Dromed, while the Doom 3 engine renders every polygon in every "area" that's in your view - so if there is a rooftop blocking your view, the engine still renders every poly on the opposed side.)
I have learned my lessons, though, and my next FM will run better and look better.
Thanks for your comment, Neb! [/edit]