How to reproduce:
1. Open the attached project "resRepro"
2. Open the scene "reproScene" and enter Play mode (Assets/reproScene.unity)
3. Add a second Game view window (Right-click on the Game tab > Add Tab > Game)
4. In the second Game view's Control Bar, set the Display to "Display 2"
5. Drag out the second Game view window as a standalone window and minimize it
6. In the first Game view, inspect the FPS being displayed
7. In the Hierarchy window, select the GameObject "Camera 1"
8. In the Inspector window, disable the Camera component's "Allow Dynamic Resolution" Rendering property
9. In the first Game view, inspect the FPS being displayed

Expected result: Framerate doesn't change
Actual result: Framerate drops by 40%

Reproducible with: 10.1.0 (2020.2.0b8), 10.7.0 (2020.3.23f1), 12.1.1 (2021.2.4f1), 13.1.1 (2022.1.0a16)
Not reproducible with: 7.7.1 (2019.4.33f1), 10.0.0-preview.27 (2020.2.0b8)

Note: The issue is not reproducible when setting the Dynamic Resolution type to "Software" in the HD Render Pipeline Asset