The cloud images from https://cloud.centos.org/centos/7/images/ contain a /etc/resolv.conf
with a spurious nameserver 10.0.2.3
entry. According to the anaconda log, this was set by DHCP during the building of the image. For most users, this entry doesn't corresponed to an existing nameserver and will slow down ssh connections, since inverse name lookups have to time out on the spurious server.
This can be fixed using virt-sysprep
from libguestfs, e.g.:
virt-sysprep -a CentOS-7-x86_64-GenericCloud-1809.qcow2 --delete '/etc/resolv.conf'
You probably also want to disable the GSSAPI authentication in ssh connections, as this is another item that can cause delays opening ssh connections. The full command for that is