Scene view lags while painting height when brush size is set to maximum

Steps to Repro:
1. Open attached or create a new project
2. Hierarchy > right click
3. 3D object > terrain
4. Select raise or lower terrain from paint tab
5. Paint height on terrain

Actual Result:
1. Scene view lags a lot, making hard to paint and smooth height. Specifically while working with large terrain or with multiple terrains
2. Lag becomes worse when painting edges of the 2 connected terrains

Occurring with:
2018.3.0a6 (13fac50c9eea) , 2018.3.0a5, 2018.3.0a4

Note:
New Terrain tools are introduced from alpha 2018.3.0a4

Environment:
Windows and Mac