Remote Desktop 6.0 leverages a new feature called auto-tuning for the TCP/IP receive window that could be causing the trouble. Window Auto-Tuning could have issues on some networks, and cheap SOHO routers. I fixed typing the following command on the server I was not able to connect:

netsh interface tcp set global autotuninglevel=highlyrestricted

after this I rebooted the server and I was able to work again.

http://blog.tmcnet.com/blog/tom-keating/microsoft/remote-desktop-slow-problem-solved.asp