Hello!
After login in SDDM, there is always a delay of about 30 seconds where nothing happens (no HDD movement) then KDE finally loads.
It's a fresh install. It seems that the problem persists with or without the NVIDIA proprietary driver (nvidia-390xx).
I deleted ~ /.Xauthority, kwallet-pam package, enable DRM kernel mode setting for NVIDIA : nothing change.
Here is the results for journalctl -b -u sddm :
juil. 02 20:42:26 arnaud-pc systemd[1]: Started Simple Desktop Display Manager.
juil. 02 20:42:27 arnaud-pc sddm[472]: Initializing...
juil. 02 20:42:27 arnaud-pc sddm[472]: Logind interface found
juil. 02 20:42:27 arnaud-pc sddm[472]: Starting...
juil. 02 20:42:27 arnaud-pc sddm[472]: Adding new display on vt 1 ...
juil. 02 20:42:27 arnaud-pc sddm[472]: Loading theme configuration from ""
juil. 02 20:42:27 arnaud-pc sddm[472]: Display server starting...
juil. 02 20:42:27 arnaud-pc sddm[472]: Running: /usr/bin/X -nolisten tcp -auth /var/run/sddm/{2bbbde4a-b3ab-400d-a38b-0fd31305e050} -background none -noreset -displayfd 17 -seat seat0 vt1
juil. 02 20:42:29 arnaud-pc sddm[472]: Setting default cursor
juil. 02 20:42:30 arnaud-pc sddm[472]: Running display setup script "/usr/share/sddm/scripts/Xsetup"
juil. 02 20:42:30 arnaud-pc sddm[472]: Display server started.
juil. 02 20:42:30 arnaud-pc sddm[472]: Socket server starting...
juil. 02 20:42:30 arnaud-pc sddm[472]: Socket server started.
juil. 02 20:42:30 arnaud-pc sddm[472]: Loading theme configuration from "/usr/share/sddm/themes/midna/theme.conf"
juil. 02 20:42:30 arnaud-pc sddm[472]: Greeter starting...
juil. 02 20:42:30 arnaud-pc sddm[472]: Adding cookie to "/var/run/sddm/{2bbbde4a-b3ab-400d-a38b-0fd31305e050}"
juil. 02 20:42:30 arnaud-pc sddm-helper[522]: [PAM] Starting...
juil. 02 20:42:30 arnaud-pc sddm-helper[522]: [PAM] Authenticating...
juil. 02 20:42:30 arnaud-pc sddm-helper[522]: [PAM] returning.
juil. 02 20:42:30 arnaud-pc sddm-helper[522]: pam_unix(sddm-greeter:session): session opened for user sddm by (uid=0)
juil. 02 20:42:30 arnaud-pc sddm[472]: Greeter session started successfully
juil. 02 20:42:30 arnaud-pc sddm[472]: Message received from greeter: Connect
juil. 02 20:42:43 arnaud-pc sddm[472]: Message received from greeter: Login
juil. 02 20:42:43 arnaud-pc sddm[472]: Reading from "/usr/share/xsessions/plasma.desktop"
juil. 02 20:42:43 arnaud-pc sddm[472]: Reading from "/usr/share/xsessions/plasma.desktop"
juil. 02 20:42:43 arnaud-pc sddm[472]: Session "/usr/share/xsessions/plasma.desktop" selected, command: "/usr/bin/startkde"
juil. 02 20:42:43 arnaud-pc sddm-helper[573]: [PAM] Starting...
juil. 02 20:42:43 arnaud-pc sddm-helper[573]: [PAM] Authenticating...
juil. 02 20:42:43 arnaud-pc sddm-helper[573]: [PAM] Preparing to converse...
juil. 02 20:42:43 arnaud-pc sddm-helper[573]: [PAM] Conversation with 1 messages
juil. 02 20:42:43 arnaud-pc sddm-helper[573]: [PAM] returning.
juil. 02 20:42:43 arnaud-pc sddm[472]: Authenticated successfully
juil. 02 20:42:43 arnaud-pc sddm-helper[573]: pam_unix(sddm:session): session opened for user arnaud by (uid=0)
juil. 02 20:42:44 arnaud-pc sddm-helper[573]: Starting: "/usr/share/sddm/scripts/Xsession \"/usr/bin/startkde\""
juil. 02 20:42:44 arnaud-pc sddm[472]: Auth: sddm-helper exited successfully
juil. 02 20:42:44 arnaud-pc sddm[472]: Greeter stopped.
juil. 02 20:42:44 arnaud-pc sddm[472]: Session started
I'm running out of ideas.
Thank you for your help!