I have this problem also,(which was also posted here
https://forum.siduction.org/index.php?topic=6826.msg55567#msg55567) this is experimental, I have a regular debian install (experimental also) with the same exact packages installed, but, no lag/wait time to login.
I have purged mythtv, mariadb, masked network-manager.wait-online.service, etc, to no avail.
I am going to make a new build and start from scratch, not today, I had a long day at work and I am cranky and tired
systemctl status sddm.service
● sddm.service - Simple Desktop Display Manager
Loaded: loaded (/lib/systemd/system/sddm.service; enabled; vendor preset: enabled)
Active: active (running) since Mon 2017-09-18 09:32:15 EDT; 30min ago
Docs: man:sddm(1)
man:sddm.conf(5)
Main PID: 800 (sddm)
CGroup: /system.slice/sddm.service
├─800 /usr/bin/sddm
└─969 /usr/lib/xorg/Xorg -nolisten tcp -auth /var/run/sddm/{051198af-358a-4fe6-88f5-716f1dd1863f} -background none -noreset -displa
Sep 18 09:32:15 x1 systemd[1]: Starting Simple Desktop Display Manager...
Sep 18 09:32:15 x1 systemd[1]: Started Simple Desktop Display Manager.
Sep 18 09:32:28 x1 sddm-helper[1181]: pam_unix(sddm-greeter:session): session opened for user sddm by (uid=0)
Sep 18 09:33:00 x1 sddm-helper[2714]: pam_unix(sddm:session): session opened for user piper by (uid=0)
systemd-analyze critical-chain
graphical.target @49.974s
└─multi-user.target @49.974s
└─mythtv-backend.service @33.992s +5.105s
└─mariadb.service @23.901s +10.089s
└─network.target @23.893s
└─NetworkManager.service @18.244s +5.646s
└─dbus.service @16.962s
└─basic.target @16.953s
└─sockets.target @16.952s
└─cups.socket @16.951s
└─sysinit.target @16.798s
└─systemd-timesyncd.service @15.494s +1.303s
└─systemd-tmpfiles-setup.service @15.372s +93ms
└─systemd-journal-flush.service @1.746s +13.622s
└─systemd-remount-fs.service @1.457s +242ms
└─systemd-journald.socket @1.425s
└─-.mount @1.414s
└─system.slice @1.423s
└─-.slice @1.413s
systemd-analyze blame | head
13.622s systemd-journal-flush.service
10.874s mythtv-status.service
10.089s mariadb.service
8.418s udisks2.service
6.124s dev-sda10.device
5.646s NetworkManager.service
5.105s mythtv-backend.service
5.056s loadcpufreq.service
5.010s apache2.service
4.531s disks-disk1part5.mount