I experienced a several second jank while loading a Google doc, which I profiled. The profiler points to JS. Here is a link to the profiler profile. http://people.mozilla.com/~bgirard/cleopatra/?report=2d39446805eacee7d5baabe959a6924cb566f599
I think this has been fixed through another bug a while back. I can't reproduce this. Please re-open if you still get this Lawrence.
I haven't noticed this type of jank anytime recently.