There is a website [1] with more informations and talks about "jank".

[1] - http://jankfree.org/

I use/have used the term 'janky' before to describe similar kinds of things or things that seem off or strange.

Grew up in the midwest fwiw

(I'd guess it's originally related to the concept of "jerk" in physics. http://en.wikipedia.org/wiki/Jerk_%28physics%29 )

It's been used in this context since at least 2010 by the Chrome team: http://en.wiktionary.org/wiki/Talk:janky

The Firefox team definitely used the term with this usage, since at least 2011.

We (at Empirical) use janky to describe animations that aren't butter as this author uses it. Not sure how standard but I suspect many people (native speakers) will understand when in context.

No, usually the term I have heard used for this is jitter.