DHCP Server in Windows Server 2008 R2 has been enhanced and tested to deliver far higher levels of performance and scalability than in previous releases of Windows. The cornerstone of the performance improvement in the DHCP server in the latest release has been aggressive caching of the lease records in database. A bit of context to let you see how the cookie crumbles: DHCP server uses Jet database engine also referred as Extensible Storage Engine (ESE). ESE provides a facility where the whole or part of the database can be cached in memory to improve lookup performance and to reduce dependence on expensive file IO which can drain performance.
Full Article: DHCP Team