Running Debian Unstable GNU/Linux with a custom 18.104.22.168 kernel, here's an analyzis of and hints for improvement of Gnome startup time.
Here's the intro:
The time it takes to log in to GNOME has not improved much in recent years and does not compare favourably with other systems, especially closed-source systems. This work attempts to determine what causes GNOME startup to be slow and what can be done to address the problem.
You can read the whole story here at Gnome.org.