Adding terrain to the scene makes Editor (and whole computer) extra slow

2) How can we reproduce it using the example you attached
Regression - the same works perfectly in 5.0.0b13

1 Create new project
2 Add terrain to the scene
3 Try to play with terrain a little (create a mountain, try to move to different parts of the terrain)

Actual results: Editor (and whole computer) becomes extra slow

4 To crash Editor try to change Terrain size(in terrain settings), for example Terrain Width - set 50

Interesting thing - the CPU usage and RAM usage is ok, but whole computer is extra slow