To reproduce:
1. Open attached project;
2. Open "test" scene;
3. In the scene view, select cube and then sphere a few times.

Expected result: game objects get selected instantly after mouse click.
Actual result: there is a delay between the mouse click and the selection made in the scene.

Note: The more DLLs your project contain, the slower the GameObject (with a material) will be.

Reproduced with: 2019.2.0b1, 2019.2.0b10, 2019.3.0a5, 2019.3.0a11.