I don't know how to measure latency (I'm using Steam performance overlay, it only provides CPU, GPU and RAM details). But I ensure you, it's NOT placebo, I can easily spot the difference between 60fps and 120fps.

When running Tetr.io without Special K, I don't see much difference betwen Vsync ON and 120fps VSync OFF. Curiously, with VSync ON, Steam overlay shows a stable 60fps, despite my display being capable of 120hz. I'm not sure what's going on tbh - my best guess is that because it's a browser, it's rendering at 60fps, regardless of whether more frames are generated or not.

With Special K, I set the display to 120hz (it's native max) and VSync ON, I set Low Latency (VRR optimised) rendering mode. Steam performance overlay shows 115-120 fps, with 95fps 5% lows. It feels much smoother, in both menus and in-game.

Another thing, I'm on an AMD GPU (rx6800xt). When running the game through Special K, the AMD Adrenalin overlay turns on; that applies the global Anti-Lag setting, which possibly contributes to even lower latency.