Building light in 4.6 requires a lot more time than 4.5


after switching from UE 4.5.1 to v4.6.1, the build time for lighting of a scene increased from 3 minutes to 38 hours!

As it’s a landscape with trees, I guessed that the new “Static Lighting on Foliage” feature was responsible for this. So I reduced the “Lightmap resolution” of instanced foliage to 4, reducing the build time from 38 hours to 16 minutes.

Any further hint to return near the 3 minutes of v4.5.1?

If the difference is building lighting for foliage, then no, it’s not going to improve, since you’re rendering lighting for a lot more things. The only thing you can do to change it would be to set the foliage to not use baked lighting. 3 minutes is not a typical amount of time for building lighting, it would have always ended up being a number of hours to render. In big game productions they sometimes even have a render farm for doing light building (a feature that UE4 supports).
If you have a huge map though, consider not using lightmass, not only would the render times be possibly too high, but you might not be able to do it if you don’t have enough RAM. And, then if it were successful you could end up with a game level that takes a huge amount of storage space just for lightmaps.