After adding grass to the terrain - the whole Editor becomes very slow (Gfx.WaitForPresent)

1. What happened
When adding grass to the terrain - the whole Editor becomes very slow and Gfx.WaitForPresent call takes 200ms (use Profiler to check that). After removing grass (by pressed Shift) - the Editor becomes to normal behaviour
This is regression - doesn't reproduce on 4.6.1.p1, and reproduces on 5.0 versions(checked: for 5.0.0b8 - you need to add more grass, for 5.0.0f2 - you need to add less grass(see attached video))
2. How we can reproduce it using the example you attached
1 Open attached porject and scene "terrainGrassSlow"
2 Notice that Editor is very slow
Please see attached video for more info
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to reproduce this on new project
1 Create new project
2 Add terrain and change its size to 20x20, start adding grass(for example use grass texture from standart assets) to it while editor start be very slow