Manually downgrading a few hundred packages one-by-one and rebooting each time was not an option so here's what I did:
1. Replaced the ExecStart line in lightdm.service by: ExecStart=/usr/bin/strace -fty -P /dev/random -P /dev/urandom -o /var/log/lightdm/strace.log /usr/sbin/lightdm 2. Using a combination of the strace log and atop, found a suspicious (long running) access to /dev/random by a child process of gnome-keyring-deamon 3. Uninstalled the gnome-keyring package 4. The issue disappeared... So AFAICT, it seems that this issue is caused by gnome-keyring making blocking calls to the RNG. Possibly here <https://salsa.debian.org/gnome-team/gnome-keyring/blob/debian/master/pkcs11/gnome2-store/gkm-gnome2-file.c#L416> ? What's weird is that the gnome-keyring package was not affected by the dist-upgrade, so the behavior may have been triggered by a change in another package...
