The problem

My Thinkpad X230 running Debian 12 with XFCE + LightDM was showing the screen for around 2 seconds before actually locking it.

This issue only happens when waking from sleep by opening the lid.

The fix

After looking aroud I found a simple fix for this, we just need to specify the graphics driver on the Xorg configuration. The solution only works for laptops with Intel Graphics.

Create this file /etc/X11/xorg.conf.d/20-intel.conf and put the following contents there:

Section "Device"
  Identifier "Intel Graphics"
  Driver "Intel"
EndSection

The problem should go away after a reboot!

Source: https://www.reddit.com/r/debian/comments/wxgjlq/lightdm_lightlocker_shows_screen_on_resume/