I actually do not think its just you having the problem,
Ever since they launched that new "HD 1080" Streaming content, There has been horrendous buffering for videos, Sometimes the little circle deal just spins forever, Sometimes the video Doesn't play at all (lolz on the inside here, The Ad's still play perfect).
But not only that, The website itself lags for me when going to the home page, or checking messages or whatever.
I've tried it on Firefox Internet Explorer and Google chrome, All three do the same thing, So does both of my laptops, so i know its not my gaming computer with a problem, That thing loads webpages as fast as you can open them.
Next time your lagging out on youtube, Try opening another window and going to another website... to see if your actually lagging.. or its just youtube itself. (which it seems to be I.M.O)
you can try and clear out your temp folders, history and caches.. like i did, but that just made everything else run better... youtube..its still slow