What fixed the problem for me, was going to www.youtube.com/html5 and opt-out of the HTML5 video player which I opt’ed-in for a long way back, since I’m not a big Flash fan.
This solved it for me, although it means going to the Flash-based player, at least YouTube now plays smoothly in Safari again. When the next Safari update is out, 5.1.8 I assume, I’ll try to see if I can enable HTML5 again and hopefully it will work like it used to, before the 5.1.7 update.
If you enjoy streaming videos online, and you enjoy films you may be interested in the streaming sites such as LOVEFiLM and Netflix. It can be quite difficult to compare the two, so you may want to learn more about lovefilm vs netflix by visiting www.netflixvslovefilm.co.uk to find out more information. Afterwards you can check out if the companies have any free trials that you can take advantage of to find out more information.