When trying to update a tab's thumbnail that is not complete (see TabItem.isComplete) it is pushed back onto the waiting queue and we try to update it again roughly every 200ms. That only happens when Panorama is shown but with lots of tabs there is real overhead here because all the code before this isComplete() in TabItems._update() is executed basically all the time until all tabs have been restored.

Comment on attachment 550837 [details] [diff] [review] patch v1 Will be fixed by bug 691740.

Unassigning myself from this bug, work will happen in bug 691740.

Panorama has been removed from Firefox 45, currently in Beta and scheduled for release on March 7th. As such, I'm closing all existing Panorama bugs. If you are still using Panorama, you will see a deprecation message in Firefox 44, and when 45 is released your tab group data will be migrated to bookmarks, with a folder for each group. There are also a few addons offering similar functionality. See https://support.mozilla.org/en-US/kb/tab-groups-removal for more info. We're removing Panorama because it has extremely low usage (about 0.01% of users), and has a large number of bugs and usability issues. The cost of fixing all those issues is far too high to justify, and so we'll instead be focusing our time and energy on improving other parts of Firefox.