Hint: if you want to check if this answer will solve your issue, execute ifconfig eth0 mtu 1400 on the instance and check the speed.
Edit the file: /etc/quantum/dhcp_agent.ini and set:
dnsmasq_config_file=/etc/neutron/dnsmasq-neutron.conf
Create and edit file /etc/neutron/dnsmasq-neutron.conf and put this inside:
dhcp-option-force=26,1400
Restart DHCP:</