I just did a fresh install of testing with XFCE. After entering the login credentials the screen was black for 30-60 seconds before the desktop environment showed up. Assuming a bug in XFCE, I performed another fresh install with MATE, but a similar effect occured: after login the background image is shown for 30-60 seconds before the desktop is fully loaded.
A look into /var/log/lightdm/lightdm.log showed that the delay happens before VT is activated. The log file is attached.
Might this be a bug in lightdm or could a misconfiguragtion from my side cause this issue?