Have you used the TV for console gaming? The PC is fundamentally no different to the consoles and so the lag will be the same.
But look for the 'game mode' low latency setting on your TV, because switching that on is a good idea (and if your eyes didn't pick up on the abhorrent visual issues that 120Hz mode creates then you definitely won't notice any potential loss in image quality that game mode could add).
Look for a game or PC mode, additionally turn off all forms of image enhancement (noise reduction, edge enhancement, contrast adjustment etc). Do the same thing on your receiver if your passing through one.