Simple enough to implement but only required in the “hottest” of environments where thousands of requests are being made per second. From within the Storefront :
Specify the maximum number of concurrent requests to send to different Delivery Controllers. The default is 0 (No Limit).
Use the Advanced Settings task, select Maximum concurrent enumerations, enter a number, and click OK.
For more information on this and also a great overview of what goes on at logon take a look at Bas’s guide, really good read.