There's no way to change playback speed by URL arguments.
Anyway, if you're working with HTML, you can take advantage of the YouTube Player iFrame API.
Here's how to configure your player with all the JavaScript: https://developers.google.com/youtube/iframe_api_reference#Getting_Started
And here's the function you're looking for to set playback speed: https://developers.google.com/youtube/iframe_api_reference#Playback_rate
So you can edit your onPlayerReady function like this:
function onPlayerReady(event) {
player.setPlaybackRate(2); // This is what you're looking for
event.target.playVideo();
}
You can of course pass on step 5 of the documentation as this will stop your video from playing after six seconds.
If you have trouble setting that up, I'll edit a JSFiddle later (couldn't do it at work as my Flash plugin won't launch).
Update :
Here's the JSFiddle working fine with this code exactly: http://jsfiddle.net/jpreynat/e11oy0eu/