By default rdesktop try to give you the best image quality which is fine on a LAN but inappropriate in other case. But using some options you can change this behaviour:
-xdisable theming-mdisable mouse motion events-zto enable compression-r soundffdisable sound-Penable bitmap caching
So now i use the following command:
rdesktop -u username -f -m -z -x m -P -r soundff -0 myip