Computer - why does a video take so long to buffer, it starts then it buffs, starts then buffs?

The video will take long to buffer if the video you are watching is very big or you have a slow internet if you have a slow internet get a faster one but if you already have a fast internet then the video must be for 5 or 6 or 4 minutes so do something else while it buffers like reading the comments or learning another language or go to the dictionary and find meanings of your words or E.T.C