x11vnc Headless on Ubuntu is very slow until monitor connected

I’m running x11vnc on Ubuntu as a service at startup. It generally works fine, but I want to run it headless. If I start the server with a monitor connected but off, VNC connects and responds quickly, however if I restart the server without a monitor connected (I want to remove the monitor from my desk) then it’s painfully slow to use – like 5 fps response, until I connect a monitor again and then it’s fine.

Here’s the content of my service file

[Unit]
Description=Start x11vnc at startup
After=multi-user.target

[Service]
Type=simple
ExecStart=/usr/bin/x11vnc -display :0 -geometry 1024x768 -auth guess -forever -loop -noxdamage -repeat -rfbauth /etc/x11vnc.pass -rfbport 5900 -shared -o /var/log/x11vnc.log

[Install]
WantedBy=multi-user.target

Be really grateful if someone can point out where I’ve gone wrong with this.
Thanks