How to reproduce:
1. Open the attached project
2. Download FPSDisplay.cs script and place it to Assets folder
3. Open test scene, create emply game object and attach script to it
4. Build and run test scene
5. Test it in windowed mode 1680x1050
6. Notice FPS written in the left top corner

Comparing results in Unity 5.3.6p1 and 5.4.0f3 performance got worse around 20%, in Unity 5.5.0a5 it got worse even 70%

Notes:
- Reproducible only on Windows; not reproducible on OSX;
- Reproducible only on high-end machines with powerful GPU;
- Bug may have significant impact on users in 5.5.