YouTube offers the ability to change the speed of a video using Safari. On the right side of the video there is a gear icon, clicking it brings up a sub-menu, in that menu there are other options, one of them is a speed option, clicking that will give you the ability to slow the video or speed it up.
And honestly speaking, watching the videos on YouTube rather than downloading them directly supports Shapr3D. As much as I can appreciate offline viewing too, I prefer to watch them through YouTube to help support the product. This of course is just my opinion.