I was also having this problem in Windows 10, GTX 1080 graphics card, and LG 2k monitor 165Hz with GSync.
The game would look like it is lagging but it was actually skipping every 2nd frame and locked at 60 fps.
When I realized this I did some testing and could replicate the problem.
I could replicate the problem and the solution, GSYNC not working as intended when the 7 Days to Die window is active in Windows 10 regardless if the window was full screen or windowed mode.
Here is the my fix:
*Disclaimer, it helps to have the Steam FPS counter turned on while troubleshooting.
[A] Monitor Technology = set to Fixed Refresh
Preferred Refresh Rate --> set to Application-Controlled
[5] Alt-Tab back to the 7 Days to Die window, making it the active window again, and confirm the setting change has resolved the FPS "lag"

From what we've observed between my friends this only seems to be a problem with Windows 10 and nVidia's GSync technology.
If this works for everyone in the Windows 10 with GSync environment, please sticky.
Thanks.
The game would look like it is lagging but it was actually skipping every 2nd frame and locked at 60 fps.
When I realized this I did some testing and could replicate the problem.
I could replicate the problem and the solution, GSYNC not working as intended when the 7 Days to Die window is active in Windows 10 regardless if the window was full screen or windowed mode.
Here is the my fix:
*Disclaimer, it helps to have the Steam FPS counter turned on while troubleshooting.
- [1] Start the 7 Days to Die game and nVidia Control Panel
[2] In the nVidia Control Panel --> Select Manage 3D Settings --> then select Program Settings
[3] Choose the program 7 Days to Die 7daystodie.exe
[4] Change the following settings:
[A] Monitor Technology = set to Fixed Refresh
Preferred Refresh Rate --> set to Application-Controlled
[5] Alt-Tab back to the 7 Days to Die window, making it the active window again, and confirm the setting change has resolved the FPS "lag"

From what we've observed between my friends this only seems to be a problem with Windows 10 and nVidia's GSync technology.
If this works for everyone in the Windows 10 with GSync environment, please sticky.
Thanks.