systemctl command line completion is very slow

Suppose I type:

systemctl status smart[tab][tab]

and I get nothing. One second, two seconds... so I insist and hit tab several
times, and after several seconds I get (one line per second):

Isengard:~ # systemctl status smart
smartcard.target              smartd.service smartd_generate_opts.path    
smartd_generate_opts.service
Isengard:~ # systemctl status smart
smartcard.target              smartd.service smartd_generate_opts.path    
smartd_generate_opts.service
Isengard:~ # systemctl status smart
smartcard.target              smartd.service smartd_generate_opts.path    
smartd_generate_opts.service
Isengard:~ # systemctl status smart

and then I can type "d" and enter.


Why is it that slow? It is an M2 disk. ie, SSD technology. It responds
instantly when completing a normal command:

Isengard:~ # smart
smart_agetty  smartctl      smartd
Isengard:~ # smart


And that system is not busy at all, load 0.20. I can replicate the issue on
other machines, and has been so probably for a year or so. Google "systemctl
completion slow" finds reports on other distributions and several media.

<https://bugzilla.redhat.com/show_bug.cgi?id=1491668>
<https://bugs.launchpad.net/ubuntu/+source/bash-completion/+bug/1471723>

and:
<https://github.com/systemd/systemd/issues/7185>