I've been seeing this for at least a few months, finally got around to filing a bug on it (although I'm rather surprised I couldn't find one already... maybe I just couldn't find it):
My browser session that I restore on every nightly update has two windows; one of those windows has 24 (currently) pinned tabs and 2981 tabs total. Session restore will restore all the pinned tabs and the active non-pinned tab. (The other window has 3 tabs in it.) Restoring the window clearly loads three tabs at once, so once one completes loading, then the next pinned tab starts loading, from left to right, visibly (based on the progress indicators).
While the pinned tabs are restoring, the awesomebar doesn't work at all. Showing the dropdown appears to be delayed until the moment when there are only 2 pinned tabs currently loading.
So if I try to type, say "query" in the awesomebar to get the Bugzilla search page while the pinned tabs are restoring... nothing happens, and it just sits there for 30+ seconds while all the pinned tabs load, until there are only 2 left to finish loading, and then, almost immediately, the search results pop up.