its probably your browser, i suggest to update it or reinstal it..
other problem could be a specific website where you watch videos, it could be bugged or smething..
another problem video drivers, and your computer parameters,maybe your computer is very weak and when you try to load something at full screan computer can lag badly...
(but this is not realistic problem... yes it could be but its not common)
other problem: you have many viruses in your system..
and one more thing, maybe its normal, sometimes when i watch video and try to move the some other program window on there, sometimes video becomes white, but that is temporary lag..
also i had a problem, when iam watching video and moving my mouse on the same time, video freezes.. that was coused by registry junk, i recomend you to scan your computer with registry cleaner, there is advanced system care suit (its free, yet limited) i think i used it for registry cleaning and it helpt...
good luck on that
