Metrics of performance benefit
On Windows 9x computers, it was possible that a very large registry could slow down the computer's start-up time. However this is less of an issue with NT-based operating systems (including Windows XP and Vista), due to a different on-disk structure of the registry, improved memory management and indexing. Furthermore, versions of Windows prior to Server 2003 may fail to start up, if the registry and kernel files are unable to fit within the first 16M of memory. Slowdown due to registry bloat is thus far less of an issue in modern versions of Windows.
Conversely, defragmenting the underlying registry files (e.g. using the free Microsoft-supported PageDefrag tool), rather than attempting to clean the Registry's contents, has a measureable benefit and has therefore been recommended in the past by experts such as Mark Russinovich
. (A form of defragmentation capability is built directly into Windows since Vista.[clarification needed])
The Windows Performance Toolkit is specifically designed to troubleshoot performance-related issues under Windows, and it does not include Registry cleaning as one of its optimizations.[