If the Package Manager UI window is open when entering Play Mode in the editor, it will attempt to refresh the packages list. This causes a major hitch upon entering play mode.

1. Open Unity
2. Open the attached project
3. Open SampleScene.scene
4. Make sure the Package Manger UI window is open via Window > Package Manager
5. Enter Play Mode

Expected Result:
The editor enters play mode and the scene plays without any issues.

Actual Result:
When the editor enters playmode, Package Manager refreshes the package list. This causes a big pause, which can be seen in the Game window as the cube stops spinning until the Package list has finished loading.

Tested on Windows.

Reproduced in 2019.1.0b7 with Package Manager UI 2.1.1

Cannot reproduce in 2018.3.7f1 with Package Manager UI 2.0.7