Dell XPS 9550 Mouse Lag only over Windows Desktop
Problem: Mouse lags when going over just the Windows desktop
Hardware: Dell XPS 9550
Software: Windows 10
Solution: Task Manager -> Startup tab -> Disable Nvidia Capture Server Proxy; Restart your computer.
Edit: More recently, I also had to uninstall Nvidia GeForce Experience
Note: This mouse lag occurs when programs that use hardware acceleration are open. For me, programs that would cause this issue included Google Chrome and Lync (Skype for Business). You can disable hardware acceleration in Google Chrome settings which would resolve the issue. Lync does not have this feature. By disabling Nvidia Capture Server Proxy, I no longer receive mouse lag on the windows desktop / wallpaper.