After much trial and error, I managed to make a map with the new terrain builder tools and export it into arma 3.
I noticed a peculiar performance related thing. It's best if you check the pics:
The map has zero (0) objects, and yet the increased object view distance drops performance by a whopping 91%, from 81 to 8 FPS.
The data structures handling drawable objects don't seem to be very efficient.
Could be that this is caused by some map parameters, but I don't know what could do that.
The map has a 4096x4096 heightmap with 4.5 cell size, making it roughly 18 km x 18 km.