Zooming an SVG with Acrobat vs. Slug Rendering Library

I watched the first video and thought "huh, that looks pretty fast, I guess the second video is the one failing ridiculously.."

Then I watched the second video and thought "heh, that looks pretty smooth and fast, I guess they both work."

I'm not sure what I'm supposed to be watching here? Is it that one has a smooth zoom while the other zooms in steps?