Hiya.
Firstly, I suggest you give us your internet speed:


Step 1. Go to speedtest.net
Step 2. Pick your closest server.
Step 3. Run the test.
Step 4. Give us the results.


If that deems to be irrelevant to your problem, give us your specs and tell us what browser you use.

It's probably because you don't have a decent gpu or integrated gpu.

Update your gpu drivers(if you have a discrete one)
Update your adobe flash

Well, actually sometimes youtube can get a bit screwy and slow vids down even if everything is top-notch on your part.
But if this happens to you all the time then yeah, it's eighter GPU or bandwidth related. Olso make sure there aren't any programs updating in the background when it slows down.