Why do videos I watch load so slowly?

Your Internet line is overloaded or too slow.  If it's overloaded turn off and don't use anything else but the TV that's on the Internet.  

If it's too slow to start with, you'll need to upgrade and get something faster.  I'm currently downloading at 10 Mps which is just barely fast enough.  On occasions I get the same thing you do...the load and run, load and run,.....  I probably should upgrade my speed, but hey I'm a cheapskate and to a point my streaming works well most of the time.

How fast is your internet speed. I had 50 mbps and I get some slowing down here and there. 

How many people share your internet when you’re viewing videos. Think of the internet as a street. If it’s just you, you can go at the speed limit. If another person joins, then you’ll have to slow down a little bit to avoid an accident. If you have 5 more people then you need to really slow down. 

Not only do you have to worry about how many people use your internet but how many people such as neighbors that use the hub. If you view during dinner time when there are more people at home, it’ll slow down because more people are using the internet street.