Hi,
I was having the same problem.
Two things seemed to help, firstly doubling my ram from 8 to 16gb and secondly adjusting the grass meshes to have more quads so the instance count was lower per kilometer of terrain.
You can also lower the number of instances per kilometer within the landscape grass type, this helped massively but taking it too low made the grass appear in “clumps” which I guess can be made to look natural but didn’t look right in my game.
After the tweaks landscape appears within 2 seconds and appears quicker in a standalone build.
I am using 4.8.2.
Let me know if this helps!