Subtractive VS Additive
I did the test, and found out why my lightning build was sooo long even though I only had one light. If there's one thing I would recommend to anyone here who use the new engine, is to completely drop the idea of creating a level using the subtractive mode. Period.
For some results:
Additive build time: 1.5 - 2.5 Seconds.
Subtractive build time: 8 - 10 Seconds.
If you're sceptic, You can try it if you want, the larger the level is, the worse it gets.
I tried it with a larger BSP only level I was working on, it was originally build on subtractive but after realizing that my light build went over 60 seconds, it started to drive me insane. Moving to Additive saved me so much time.
What should I do if I only use the Subtractive method then?
This question gets a simple answer. Go click on the "New level" button and chose Additive.
Once you're in, Set your red builder brush to be large enough and click on "CSG: add". Once you'll get into subtracting, you'll be able to work inside the cube. If you need windows/skies you'll have to bring one of the cube's face to the edge of your subtracted brush.
That way you can map the old way style, hope that helped. Just keep all of this in mind.
I already have a level I'm working on using the Subtractive mode. Can I port it to Additive?
Yes. You will have to do the same as I mentioned above for the "CSG: add" cube, copy/paste your entire level. I didn't ran into any kind of massive issues, but the after-build log was huge when I only build my geometry. I recommend you do a "build all" right after you pasted your level to avoid facing this situation.