Heres the fix (well, this worked for me).

This problem existed in RC1 and to some extent RC2 too. I did report it, and it's partially fixed but still not good. Microsoft at the time blamed my Cisco router.. Anyway, I get really poor intermittant network performance. It turns out this is due to a new addition in the TCP/IP stack called Receive Window Auto-Tuning Level.

To check the status of this feature, run "netsh interface tcp show global" at a command prompt.

You will see something like:

Querying active state...

TCP Global Parameters
----------------------------------------------
Receive-Side Scaling State : enabled
Chimney Offload State : enabled
Receive Window Auto-Tuning Level : disabled
Add-On Congestion Control Provider : none
ECN Capability : disabled
RFC 1323 Timestamps : disabled

In this example, you can see I have it disabled. (My network performance returns to normal with it off!).
To disable it, run: "netsh interface tcp set global autotuning=disabled"
To enable it, run: "netsh interface tcp set global autotuning=normal"
(Both these last two should be run from an Administrator Command Prompt, not a "normal" command prompt).