I can confirm the same issue, according to what I've checked it was a limitation of ultravnc.
According to some tests for a similar tool I've made, depending of how old a vnc viewer is (beacause it does not support recent vnc protocols) the performance can be improved, so tweaking the ini could improve the refresh performance. I don't have time to dedicate for my tool dough...
There is also a post regarding x86 and x64 architectures that can cause such behavior, but in my case I see poor refresh rate between windows xp x86 computers.
http://forum.ultravnc.info/viewtopic.php?p=80683