Enabling vsync on my machine (R9 290X @17
.1.1, i7-4790K @ stock, 32GB RAM, Win10 v14393) absolutely obliterates performance down to sub-5-FPS levels. With it off the game runs as usual, circa 50 FPS in Ocean House 2 with no characters. Game is in 3840x2160 (native) "fantastic" quality, "fullscreen" mode (such that it is), all in-game settings at max except FXAA instead of temporal, and RT reflections off.
What other kinda info would be helpful? My monitor supports Freesync in theory, not that it ever seems to do much. Disabling it did not change the situation. I fiddled around with the settings a bit but it doesn't make much difference how they are set nor where the camera is. Vsync-ON = ~2-5 FPS. Enabling RT reflections initially gave me some crazy artifacting but it cleared up when I moved the camera.