What codec to use for fastest playback
I'd say your encoding settings are going to be more crucial, given you're limited to the codecs that Chrome can play. Unless I'm wrong you can only use MP4 / h.264 and WebM / VP8 unless you install plugins. 

Try lowering the profile of your mp4, e.g. encode using baseline profile. That will mean less compression–so larger files for the same quality or lower quality at the same size–but it reduces the processing power needed to display it.

It is worth noting that there are more hardware decoders commonly available for h.264 encoded video than there are for vp8, meaning that it's more likely to be handled at the hardware level than vp8.