Not directly related to your main question, but I suggest watching youtube video via cutetube. You save on having browser + 23544354 unneeded parts of youtube page loaded. This way, you can view (flawlessly) a one or two step higher quality videos.

Twe fact that it lags when you have conky in background seems to indicate that you're hitting performance limits with sole site + video, and conky pushes it a little over the edge.

/Estel