When using a Gsyn/Freesync monitor with unity and in windowed mode on Windows 10 a huge drop in framerate can occure, causing the framerate to drop from 144fps to ~22fps in this example on a 1080ti

1. Make sure you are in an updated Windows 10
2. Make sure you are using a Gsync monitor with Gsync enabled for windowed mode in Nvidia control panel
3. Open the attatched project in unity 2018.1f1 or newer(project uses HDRP)
4. Enter play mode

Expected results: With a decent GPU you should see the framerate sit quite high or a thte max the display can do
Actual results: The frame rate will drop to ~20fps and stay there

Repo'd in 2018.1.0f1 but exists in prior versions of unity and most likely trunk

Notes:
-Resizing or maximizing/minimising helps repo as it is not quite 100%
-Doesnt require HDRP but was a good project to tax the 1080ti
-Slow down seems to come from Gfx.WaitForPresent kicking in
-Doesnt repo in fullscreen or on Windows 7