VNC FAQ-o-Matic : VNC Basics : Is VNC always this slow?
A great tip from http://perso.wanadoo.fr/samfd/esvnc/: TIP: In order to get maximum refresh rate, responsiveness and performances, it is necessary that you completely disable “Hardware Acceleration” on the machines that run WinVNC (server). It can be done in “Display Properties” Panel -> “Settings” Tab -> “Advanced…” Button -> “Troubleshooting” Tab -> “Hardware Acceleration” Tab set to “None”. This makes a big difference, especially over LAN connections and even with others VNC distributions.