I had lag on the video output over HDMI to my Rift. I had noticed it before when I connected my Computer to my home theater via HDMI. The solution: Turn off Audio over HDMI in the NVIDIA settings.
It seems like routing audio through HDMI adds a lot of lag/latency to the video output. Has anyone else the same issue and maybe a solution? For the Rift it's not a problem, but I would like to play some games on my big screen with my audio system.