A hefty amount of research indicates that, despite popular sentiment, it is not wise to set the 'DefaultTimeoutStartSec=10s' (note the 'Start' entry here)
First off, it does not help with any apparent systemd bugs, lagging daemons, programs that won't quit, or any other shutdown/reboot issues. Instead, what it will likely lead to, I am finding out, are problems with journald during boot, like described below:
systemd: Failed to start Journal Service.
systemd: Dependency failed for Flush Journal to Persistent Storage.
systemd: systemd-journal-flush.service: Job systemd-journal-flush.service/start failed with result 'dependenc
systemd: systemd-journald.service: Unit entered failed state.
In all of the relevant cases, the reason for the above error was that the user had their /etc/systemd/system.conf set to either:
By commenting out '# DefaultTimeoutStartSec=10s' or setting it to a higher value (30s) the issue was solved for all of those (I could find) with applicable issues.
Moreover, a dozen or more threads indicate that those who modified their value for 'DefaultTimeoutStopSec=10s' should re-adjust that value to at least '=30s.' This would be to ensure, especially on older machines, that a clean unmounting on shutdown/reboot is fully possible.
Not to step on anyone's advice which may contradict parts of this.
I looked at a couple of dozen threads; but this this post (below) sums up most of the pertinent stuff: