After struggling with this Nautilus’ slowness for a while, I decided to change my default as well. The simple solution that works for me in all cases without mucking with changing desktop files was to do the following two commands:
sudo mv /usr/bin/nautilus /usr/bin/nautilus.bak sudo ln -s /usr/bin/pcmanfm /usr/bin/nautilus
You can of course replace /usr/bin/pcmanfm with /usr/bin/thunar, or whatever.
If you also want to have a desktop, open /etc/xdg/autostart/nautilus-autostart.desktop and make the Exec line