Reenabled systemd-timesyncd and am back to chrony not starting on boot so at least I can reproduce at will. Here are the outputs:

$ sudo systemd-analyze critical-chain
The time after the unit is active or started is printed after the "@" character.
The time the unit takes to start is printed after the "+" character.

graphical.target @32.789s
└─multi-user.target @32.789s
  └─owserver.service @12.224s +1.060s
    └─network-online.target @12.222s
      └─NetworkManager-wait-online.service @4.481s +7.740s
        └─NetworkManager.service @4.211s +250ms
          └─dbus.service @4.007s
            └─basic.target @3.972s
              └─sockets.target @3.972s
                └─uuidd.socket @3.972s
                  └─sysinit.target @3.970s
                    └─systemd-timesyncd.service @3.840s +129ms
                      └─systemd-tmpfiles-setup.service @3.827s +11ms
                        └─local-fs.target @3.824s
                          └─run-user-1000-gvfs.mount @13.514s
                            └─run-user-1000.mount @12.399s
                              └─local-fs-pre.target @297ms
                                └─keyboard-setup.service @196ms +100ms
                                  └─systemd-journald.socket @193ms
                                    └─system.slice @193ms
                                      └─-.slice @191ms


$ sudo journalctl -o short-monotonic -u systemd-timesyncd.service -u chrony.service | head -n 20
-- Logs begin at Thu 2018-05-31 12:51:22 WEST, end at Thu 2018-07-05 18:23:07 WEST. --
[    2.756688] coulson systemd[1]: Starting Network Time Synchronization...
[    2.865804] coulson systemd[1]: Started Network Time Synchronization.
[   33.233208] coulson systemd-timesyncd[292]: Synchronized to time server [2001:67c:1560:8003::c8]:123 (ntp.ubuntu.com).
[  722.864694] coulson systemd[1]: Stopping Network Time Synchronization...
[  722.900591] coulson systemd[1]: Stopped Network Time Synchronization.
-- Reboot --
[    9.812874] coulson systemd[1]: Starting Network Time Synchronization...
[    9.949073] coulson systemd[1]: Started Network Time Synchronization.
[   40.394096] coulson systemd-timesyncd[573]: Synchronized to time server [2001:67c:1560:8003::c7]:123 (ntp.ubuntu.com).
[10529.558443] coulson systemd-timesyncd[573]: Synchronized to time server [2001:67c:1560:8003::c8]:123 (ntp.ubuntu.com).
[20405.821605] coulson systemd-timesyncd[573]: Synchronized to time server [2001:67c:1560:8003::c8]:123 (ntp.ubuntu.com).
[29239.165550] coulson systemd-timesyncd[573]: Synchronized to time server 91.189.89.199:123 (ntp.ubuntu.com).
[34935.400396] coulson systemd-timesyncd[573]: Synchronized to time server 91.189.89.198:123 (ntp.ubuntu.com).
[64222.761628] coulson systemd-timesyncd[573]: Synchronized to time server 91.189.89.198:123 (ntp.ubuntu.com).
[77531.401846] coulson systemd-timesyncd[573]: Synchronized to time server [2001:67c:1560:8003::c8]:123 (ntp.ubuntu.com).
[79656.970968] coulson systemd[1]: Starting chrony, an NTP client/server...
[79656.971139] coulson systemd[1]: Stopping Network Time Synchronization...
[79656.997189] coulson systemd[1]: Stopped Network Time Synchronization.
[79657.110021] coulson chronyd[23591]: chronyd version 3.2 starting (+CMDMON +NTP +REFCLOCK +RTC +PRIVDROP +SCFILTER +SECHASH +SIGND +ASYNCDNS +IPV6 -DEBUG)

Seems like after reboot chrony wasn't even attempted to start even though it's still enabled:

$ sudo systemctl is-enabled chrony
enabled