By default 12.04 takes advantage of hardware acceleration. The graphical interface is so on the graphics card instead of on the CPU. This should grant overall better performance and augmented visual effects on decently spec'd desktop/laptop computers.
All that comes to a cost though. Apart from the fact that complex visual effects require more bandwidth, Vino doesn't seem ready for hardware compositing (GPU acceleration) anyway.
If you have a very good connection to the computer (ie. local wired connection), you can try to disable what is called "xdamage". Open the terminal and issue the following command (as standard user, not root):
gsettings set org.gnome.Vino disable-xdamage true
This will force the server to re-send the whole screen at each refresh and not just the changed areas. It is extremely stressful for your network of course.
As an alternative to entering this command in a terminal window, you may use the dconf-editor which can be installed using the Ubuntu Software Center.
If you feel adventurous you may also try to install "CompizConif Settings Manager" and carefully reduce the number of effects (especially those related to windows and menus placement). The fewer the better for VNC.
Otherwise you can try to disable compositing altogether and login into Unity 2d you can follow these instructions, but you are of course disabling 3d effects and HW acceleration.