RDP running slow on some PC's connecting to 2003 server

Hi 

A client has a 2003 standard server. They connect to it with several accounts to access CRM and Office programs. Most user have no problems but some  have problems with the CRM software showing that it not responding on the top tool bar. When opening word excel it takes around 5 to 10 seconds for it to respond. the mouse keyboard seem to react fine. This seems to be related to the hardware. When connecting using a different PC's from the same local it works fine?

Any help would be great

Thanks

Robert.