There are several ways to throttle this. The first, and most likely to provide immediate relief is to use an IIS configuration to throttle download performance.
http://technet.microsoft.com/en-us/library/cc720543.aspx

Limiting bandwidth on a specific Web site
To limit bandwidth on a specific Web site (on Windows Server 2003)
Open IIS Manager (click Start, click Administrative Tools, and then click Internet Information (IIS) Manager).
Navigate to the Web Sites node under the local computer, select the specific Web site, right-click the node, and then click Properties.
Select the Performance tab.
Under Bandwidth throttling, select Limit the total network bandwidth available for all Web sites on this server, and then specify the maximum bandwidth in kilobytes per second (the default is 1024). You cannot specify a bandwidth lower than 1024 kilobytes per second.
Click OK.

The next option is to use BITS to throttle the download. This is a client side setting, so group policy will have to be refreshed in order for this setting to be seen.
http://technet.microsoft.com/en-us/library/cc720428.aspx

To set BITS bandwidth limitations
Start the Group Policy Object Editor (click Start, click Run, and then type gpedit.msc).
Expand Computer Configuration, then Administrative Templates, then Network, then Background Intelligent Transfer Service.
Open the Maximum network bandwidth that BITS uses (BITS 2.0) or Maximum network bandwidth for BITS background transfers (BITS 3.0) setting.
Set the transfer rate in kilobits per second that you want BITS to use (the default is 10).
Set the times at which you want to limit the bandwidth (the default is 8:00 A.M. to 5:00 P.M.).
Set the limitations to be used outside of the designated time (the default is Use all available unused bandwidth, but you may select another limitation).
Click OK.

For more information see
http://blogs.technet.com/sus/archive/2008/11/19/more-on-throttling-wsus-downloads.aspx