How to reproduce:
1. Download and open the project "1267498 (2020.2).zip" (when testing on 2019.4 download the project "1267498 (2019.4).zip" instead)
2. Open the Scene "SampleScene"
3. Select "Terrain" in the Hierarchy
4. Navigate in the Inspector to Terrain > Paint Trees
5. Select the tree "SpruceTreeA1"
6. Place the trees in the Scene on the Terrain

Expected result: the trees place smoothly without any delay
Actual result: a pop-up "Hold on SceneView.OnGUI.repaint" appears that delays the planting of the trees

Reproducible with: 2020.2.0a11 (7.5.1), 2020.2.0b6 (10.0.0-preview.27)
Not reproducible with: 2019.4.12f1 (7.3.1, 7.5.1)
Could not test with: 2018.4.28f1, 2020.1.9f1, 2020.2.0a10, 2021.1.0a1 (have skipped testing these versions for now due to the complicated nature of setting up the testing environment)

Notes:
- URP is not supported by the used package (Hazy Hills - Conifer Forest)
- When testing 10000 trees were planted as a prerequisite on a 1000x1000x1000 Terrain (noticeable delay in planting the trees appears around the ~5000 trees mark). This delay was not seen in 2019.4 and the trees would get planted instantaneously
- In 2020.2 "FloatingDust" forces the Editor to trigger "Hold on SceneView.OnGUI.repaint" when leaving and then returning to the Scene window (take notice that this only happens if the Terrain is enabled and has the trees planted from before)
- Currently the exact version this regressed in is unknown as explained in the could not test with section