To improve the performance of network connections that seem very low, especially for connecting RPC over HTTPS Outlook or Outlook Web Access, you can disable the "tuning" automatic Vista:

Go to "All Programs" then "Accessories", right-click on "Command Prompt" and click "Run as administrator".
Enter the command follows:
netsh int tcp set global autotuninglevel = disabled

Improve bandwidth

By default, Windows Vista sets aside 20% of the bandwidth.
Click on Start and then in the search box type gpedit.msc
Go to Local Computer Policy> Administrative Templates> Network> QoS Packet Scheduler
Double-click "Limit bandwidth service" and select "Off". Beware, if "Not Configured" is checked while 20% are ignored by the system. Therefore choose Off.
Click OK and then restart the computer.