rdesktop is used to login into windows machines from linux/unix machines. By default rdesktop won't enable caching. Because of this default behavior rdesktop fetches portions of the screen that weren't updated at all. This is particularly painful if your target machine is geographically situated somewhere else.
You just have to add two flags -P -z to your rdekstop command to enable local disk based caching and compression of transmission.
rdesktop -P -z [target machine ip]
I have no clue why this option was not enabled by default since it consumes very little disk space.