Reproduction steps:
1. Create a new project
2. Install Device Simulator Package via Package Manager
3. Import "FPSCounter.cs" script (attached in edits)
4. Attach the script to any GameObject in Hierarchy
5. Open Device Simulator Window (Window -> General -> Device Simulator)
6. Edit the window layout to match "Layout.png"
7. Enter Play Mode and make sure the Simulator window is invisible
8. Inspect the framerate in the Console or bottom left corner of the editor
9. Close Simulator Window

Expected result: framerate will be the same as in step 8 (read notes below)
Actual result: framerate greatly increases, meaning the Simulator window was causing the framerate to drop

Reproducible with: Device Simulator 1.0.0 - preview (2019.4.10f1), Device Simulator 2.2.3 - preview (2019.4.10f1, 2020.1.5f1, 2020.2.0b2)
Could not test with 2018.4 because Device Simulator Package is not available

Tested Windows and Mac

Notes:
1. This can also be seen in Profiler (cmd/ctrl + 7)
2. Reproducible in all package versions
3. Images and videos are attached