If you have a Linux system that was using systemd as its init system, and you changed it to something else, and now nearly everything is incredibly slow:

Remove libnss-systemd; check for and remove systemd options from /etc/nsswitch.conf.

As you will no doubt recall, that config controls the methods that the system uses for various lookups. Most things are files, which is to say the system should look in the appropriate config files. hosts is usually files followed by DNS. If you have Kerberos or LDAP or some other networked signon system, this is where it would be configured.

Who knew that systemd wanted control of that, too? Not I.