Yeah, there's some arguments about that on their bug tracker page and it looks like someone tried to change that code but it never went anywhere. If you horde tabs like I do it can still waste a lot of time to restart the browser and wait for the session to be restored.
Pretty consistent. I haven't timed it, but it certainly feels like more than 15 seconds for my 50-100 tabs. I'm not in quite as pure of a situation as that benchmark though-- I have plenty of other programs using memory and I'm waiting for the page to actually render. During some of my Firefox launches lately I also had a bunch of other applications launching at the same time since I was trying to debug a WebGL page that sporadically hangs/reboots my machine (in both Firefox and Chrome)
I believe this feature gets disabled if you enable the feature that automatically restores your previous session when Firefox is opened.