I should point out (if you are not already aware) but EVERYTHING you look at on the internet has to be downloaded to your PC FIRST, before you can view it.
For most web sites this is not a problem as the text and graphics are quite small, so they download quickly.
But video is HUGE (compared to basic text and graphics) so it can take a long time to download.
And if the web site that is feeding it is busy (like say the BBC or youtube), or if the internet network is busy (or both), then it can slow things down.
Some web sites have an option to watch in lower quality, see if that is an option as it will download quicker.