A small update on this…
The gnome-session and gnome-shell load normally (no fail whale) if I don’t have any network connectivity during the boot process… (no wired or wireless networks accessible). It still looks to me as an timing issue. I’ve done the bootchartd run again in this case (no networks) and the startup does look a bit different. gnome-session-daemon and gnome-shell still fail once (and are then respawn) but they do so much quicker, and I don’t get the timeout error in .xsession-errors anymore.
I’m attaching the new bootchart image here
By the way, the booting process in 12.1 doesn’t look that optimized to me. It doesn’t say everyting, I agree…but 19 s total cpu time in a boot of (more or less) 110 s means a lot of waiting on IO / dependencies / whatnot. Or is it that I’m not reading the graphs correctly?