" The test methodology was pretty simple - we wanted to see how each protocol (SPICE, RDP, and ICA) performed in a controlled environment.  We did this before we stress-tested the SolidICE and XenServer-based solutions, so this has nothing to do with server sizing or anything like that.  This is a simple setup with one client talking to server running a single XP virtual machine.  In each scenario, we ran through the same tests and recorded what the user actually sees as well as what the CPU utilization and network bandwidth metrics within PerfMon showed. Each of these scenarios can be viewed from within the player after it is launched."

Source: http://www.brianmadden.com/blogs/gabeknuth/archive/2009/06/18/redhat-spice-vs-rdp-vs-ica-performance-video.aspx