Comment 44 for bug 291467

I have managed to fix it for me with the following trick. Login is now is close to 1 sec.

*1* Create a file /usr/bin/my-gnome-session with the following contents:

#!/bin/sh
gnome-wm &
/usr/bin/canberra-gtk-play --id="desktop-login" --description="GNOME Login" &
/usr/lib/gnome-session/helpers/at-spi-registryd-wrapper &
/usr/lib/gnome-session/helpers/gnome-keyring-daemon-wrapper &
/usr/lib/gnome-session/helpers/gnome-settings-daemon-helper &
gsynaptics-init --sm-disable &
/usr/lib/vino/vino-server &

And make it executable:
chmod +x /usr/bin/my-gnome-session

Basically this is just all the commands from /usr/share/gnome/autostart. If you have different autostart files then you might want to edit it. Check with grep Exec *.desktop.

*2* Move all the autostart files to a temporary directory:

cd /usr/share/gnome/autostart/
mkdir tmp
mv *.desktop tmp

*3* Create a new file in /usr/share/gnome/autostart/start.desktop:

[Desktop Entry]
Name=Auto Start Scripts
Exec=/usr/bin/my-gnome-session
OnlyShowIn=GNOME;
Terminal=false
Type=Application
X-GNOME-Autostart-Phase=Initialization

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That's all. I hope this works for you as a temporary fix.