I have just bought a new notebook preloaded with win8 (and I must admit that MS has done a very good job to prevent one from easily install a linux distro) with a nvidia video card.
By default optimus would choose the built-in video card with Chrome. You have to explicitely enable the nvidia card in the driver config.
I don’t think this is covered anywhere in the doc. If this is the case and you think is worth adding, I can update the doc, just let me know where the best place would be.