Why is HD video lagging on Youtube?

Depends on your internet connection above all else.

as Randmonizer said, its mostly dependant on your internet connection.

When you are watching yourtube hd and its lagging, go to www.speedtest.net and check your connection speed.

A lot of internet connections these days are sold as (for example) 8 megabit but are actually rate adaptive. I.e. it will give your the fastest connection it can up to a maximum speed of 8 megabit. All this depends on other network traffic. If all your neighbours are also online it will suck up the available bandwidth on your exchange.

I have a DL speed of 9.41

so about 1.165 MB/s, it's not enough to stream HD video since HD video is normally 1.5MB/s.

It gets even worse when you look at Blu-Ray MB/s.

It doesn't depend on the internet-speed! What the heck?
My computer also lags in playback WHEN IT'S DONE DOWNLOADING, so...
I always wait when it's done loading, or it will stutter around anyway, which isn't nice to look at.
But it just floods the CPU to nearly 100% usage with HD-mode on.
Probably because it's a P4 èh?...