When using VNC through Guacamole performance can be very poor, taking minutes for screen updates, appearing to be frozen.

You may also see messages in the guacd / tomcat service warnings indicating screen resize occuring due to web page resizing.

To fix this add the line to disable display resizing for the vnc connection:

# /etc/guacamole/user-mapping.xml
   ...
  <connection name="PC">
    <protocol>vnc</protocol>
    <param name="hostname">192.168.1.234</param>
    <param name="port">5900</param>
    <param name="disable-display-resize">true</param>
  </connection>