Everytime you watch a youtube video online a small piece of data is stored on your computer. If you don't clear your computer out these can build up and make your puter run slow. Try downloading the free cleaner called CCleaner and it should clear things right up for you. Another reason is that you could have lots of programs running which is taking up all your RAM. Try restarting your computer and just open Youtube with nothing else open. Hope this helps.
