YouTube added an option to stop buffering a YouTube video; just right-click on the video and click on "Stop download". "The YouTube video player downloads a video as it plays. A buffer is a section of memory in your computer which allows simultaneous writing and reading of information -- on YouTube buffered section is represented by the red section of the video timeline. YouTube video player reads video information from one section of the buffer while writing to another. This kind of multitasking allows for smoother playback of video during a continuous process of downloading which’s especially helpful for slower connections," notes YouTube help center.