Reproduction steps:
1. Open project in "New Unity Project.zip"
2. Open SampleScene
3. Make sure Inspector Window is visible and nothing is selected in Hierarchy
4. Enter Play Mode
5. Switch rapidly between different GameObjects in Hierarchy and inspect framerate in Game Window (read note 2)

Expected result: framerate stays relatively the same
Actual result: framerate drops heavily for every few frames after selection (read note 1, 3)

Reproducible with: , 2019.3.0a2, 2019.3.14f1, 2020.1.0b8, 2020.2.0a10
Not reproducible with: 2017.4.40f1, 2018.4.22f1, 2019.3.0a1

Not reproducible with Windows 10

Notes:
1. When selecting an Object for the first time after entering Game Mode the entire Game Window freezes for ~0.5 s. This is reproducible in every version.
2. Similar behaviour can be seen when moving the mouse cursor above and out of bounds of any Window in Unity
3. Video is attached
4. In 2019.3.0a1 the FPS rarely drops below 50, in 2019.3.0a2 the fps drops as low as 20 very often, therefore, I concluded it to be the regression version. Also, the behaviour in note 2 is not reproducible with 2019.3.0a1 and is reproducible with 2019.3.0a2