Changing the default load limit of batch (atd)
debian stretch: Edit, update /etc/systemd/system/multi-user.target.wants/atd.service
E.g.: Change the load limit to 1 (default: 1.5):

/etc/systemd/system/multi-user.target.wants/atd.service
[Unit]
Description=Deferred execution scheduler
Documentation=man:atd(8)

[Service]
ExecStart=/usr/sbin/atd -f -l 1
IgnoreSIGPIPE=false

[Install]
WantedBy=multi-user.target

man atd