We've seen examples of a site calling element.clientWidth (or similar) inside a loop, making that site slow due to the relayout that each clientWidth call requires. It would be great for the Inspector to indicate what JS caused a relayout (or repaint) so that web developers could find and fix these problems.
One idea Hyatt, Tim, and I discussed is to show when relayouts/repaints happened on a timeline. Then you could have some way of finding out the JS backtrace that was active at the time of the relayout/repaint (if any).
If we do put this information into a timeline, it probably makes sense to unify it with the resource-loading timeline in the Resources panel. See bug 26407 for another idea which might warrant extending the resource-loading timeline to show more types of information.
See also Bug 26317 ; the referenced paper discusses general ideas regarding debugging information based on events. It would be useful to see ALL the events fired over a period of time, be able to filter those, etc.
The Timeline panel has this info.