There are a handful of Chromebooks out right now with varying hardware in them. One of the ways the entry level Chromebook keeps its price down is by having a lower end processor and video card. This can pose an issue, especially for displaying any videos in HD. If you find that your chromebook is "lagging", "chugging", or it feels like it's "buffering" all the time. It may not be your connection, it could be that the hardware isn't suited to display our HD versions.
Try downloading the SD version of our mp4 clips, or even the smartphone version. Chromebooks will play mp4's w/out having to install any extra software.