Conversely, here’s jankfree.org with a near-perfect scrolling experience:
You can detect jank using the Timeline tab in Chrome Developer Tools. Simply hit the record button and scroll, then stop the recording and look for slow frames:
You can also enable the framerate overlay in Chrome to show the live render performance:
This can all be a bit time consuming though, recently I’ve found myself spending a lot of time in the timeline view just to detect jank. The framerate meter also makes it difficult to identify exactly where in the page the jank is coming from.
The bookmarklet produces a small overlay on the top-right of the screen showing the framerates achieved while scrolling the page. It also adds a bar to the left of the page showing what framerate was achieved for that scroll movement.
There are probably a few more limitations, give it a go and let me know if you have any issues. The code is in a gist so feel free to fork.
Paul Lewis has a great live demo on youtube of debugging and fixing jank in realtime.
There are a number of resources available, jankfree.org is a great place to start.