Performance of the DNS Server service keeps decreasing under a heavy load situation in Windows Server 2008 R2
http://support.microsoft.com/kb/983534/en-us
Both samples are not affected by the described problem but may help you identify.
Some reference with around 200+ clients and 150 thin client. FSMO Master Server 2008 R2. Regular business hours 30-60 Request per Second.
Another refrence with 400+ clients Windwos XP SP3