How to reproduce:
1. Open the attached project "1285432.zip"
2. In the Hierarchy select "Test"
3. Navigate to the Inspector window
4. In "Add To Stash" click on the element list and choose a new object (click on the circle with a dot in the center)
5. Scroll down the Select Object list (with the mouse scroll or the attached scroll bar)

Expected result: The Select Object list works smoothly
Actual result: The Select Object list is severely slow freezing up for a couple of seconds with each click or scroll

Reproducible with: 2019.3.0b2, 2019.4.12f1, 2020.1.9f1, 2020.2.0b6, 2021.1.0b2
Not reproducible with: 2018.4.28f1, 2019.2.21f1
Could not test with: versions between 2019.3.0a1 and 2019.3.0b2 due to the Editor crashing with a new or existing project