Just casually waiting alongside his bro Prism
The biggest thing is that in both of these maps (as well as a portion of the Ballrace maps) there are a lot of individual actors (or objects, however you wish to call them), which all require drawcalls. This means that if there is a big section that is made of lots of tiny parts, the engine will try and calculate if all those objects need to be rendered. Sounds good, right? Well, if it's there are a lot of objects in your view, it all adds up to the render time. In the current state, all of those castles in this map are made very, very, very modularly: a model for the base wall, another for the top of the wall, another for the wall corner, another for the top of the wall corner... The result: too many drawcalls to call acceptable.
And all that for some flexible level design in the editor.
But then, Unreal Engine 4.12 came to the rescue with their actor merging feature, which allows you to merge multiple actors into one, reducing the drawcall count of all those individual wall actors to 1. The problem is: we don't work with 4.12 yet, because converting the entire project to a new version always breaks something, if not a lot. So we postponed the conversion to at least after the Casino comes out, so we don't break the casino and have to delay it more.
TL;DR: lots of individual models adding up to render time, gotta wait for UE4.12.
Other than that, it's pretty much done.