That still takes time to refetch yt page and parse for mp4 link. Smplayer can do it too, but it takes ~1 second, thats ~1 second too long for me :P. If I wasnt so lazy I would find youtubecenter code responsible for this parsing, but I am, so I simply
document.getElementById("watch8-ytcenter-buttons").children[0].children[0].href
and bam, mp4 link ready in javascript for free.
Got plenty of Linux in my life (mostly embedded, for example running my DIY TV build from recycled 32' LCD panel, universal chinese lcd controller and rasPee. Yes, I like repurposing junk). Linux is how I learned to cobble up together ugliest hacks imaginable.