Hello Neville,
SLiM is my favorite DM and the only one I use. It is light, DE independent and easily customizable. The configuration can be changed by editing /etc/slim.conf and themes are in /usr/share/slim/themes. You can use any .jpg file you like as a background by replacing the “background.jpg” located at /usr/share/slim/themes or change fonts and colors by editing “slim.theme” file. I always set SLiM to auto_login, so I don’t have to type the password every time (uncomment the lines “default_user” and “auto_login” set to “yes”). I think that the boot delay in your case is caused by saving and restoring the Xfce session and not by the DM.
Cheers