Thanks to efforts of many people in the Mozilla community, combined with tools like DTrace, we seem to be awash in performance data and awareness these days. And in the wonderment of this new visiblity, Rob Sayre and Brendan Gregg noticed that Session Restore was responsible for a significant portion of page-load time! (/me takes an anxious bow, and dodges the tomatoes)
There are a couple of opportunities for improvement, chronicled here. The first approach we're trying out is to save and restore less back-history for each tab. Easy to implement, but it wasn't the first choice since it decreased the utility of the feature... or so we thought, until Alex Polvi provided some data indicating that users very rarely go back further than 6 steps.
The change that landed today will reduce saving of back-history to 10 pages, from a default of 50. You can set the browser.sessionstore.max_tab_back_history pref to -1 if you want all tab history restored.
Comment on the bug if you're one of the <1% who go back beyond 10, and are truly disgruntled about this change. Or better yet, fix my patch to cache tab history :)