I had this issue moving to an LG CX with a 2080 Ti. Nothing worked until I disabled G-Sync for both fullscreen and windowed program, and that fixed it right up. Photoshop and G-Sync really do not mix, and there were sporadic lag issues I would encounter even before moving to a 4k display that also got fixed up by disabling G-Sync.
Instead of disabling windowed G-Sync globally, it's probably better to go to Nvidia Control Panel > Manage 3D Settings > Program Settings > Photoshop.exe and set Monitor Technology to Fixed Refresh in order to prevent Photoshop itself from trying to use G-Sync. This way Photoshop will return to its smooth glory no matter your global G-Sync settings.
I tried to change the Monitor Technology setting in the nVidia Control Panel but it didn't help me at all, however after playing around with other settings I've found one which did the trick. I've changed Vertical sync to off.