Comment 12 for bug 812104

further notes related to previous comment:

The delay present when closing the Unity-2D session (specially when using the application dash) is logged in .xsessionerrors file.
I have enabled '--debug' for gnome-settings-daemon in /usr/share/xsessions/ubuntu-2d.desktop to get more verbosity and looked at logs after the session is completely closed (switched from lightdm to vt).

these seem the more evident differences between a clean and a slow session:
(process:3926): libunity-CRITICAL **: unity_inspector_on_unity_vanished: assertion `conn != NULL' failed
g_dbus_connection_real_closed: Remote peer vanished with error: Underlying GIOStream returned 0 bytes on an async read (g-io-error-quark, 0). Exiting.