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.