Troubleshooting the database when vCenter Server is slow

vCenter Server is a database application and is therefore subject to the same issues as similar applications. These are common issues that may occur:

• Slow response when retrieving performance data, tasks, events, or inventory information.
• Only real time performance data is available.
• You see the error:
  
  Performance data is currently not available for this entity

To troubleshoot these issues:

1. If you are running a Microsoft SQL database, perform a defragmentation of the performance data indexes on the vCenter Server database. This ensures that the data can be accessed as quickly as possible. For more information, see Performing a defragmentation of the vCenter performance data indexes on a Microsoft SQL database (1003990).
2. Validate whether the vCenter Server database server has adequate resources available. Consult the vSphere Installation Guide for your version of vCenter Server, along with the vendor documentation for the database. The resolution may vary depending on the load and the database being used.
   
   It may be beneficial to separate vCenter Server and the database if they are running on the same server and if there is a large amount of activity in the environment. Resource contention can cause a noticeable decrease in vCenter Server performance.
   
   These steps may also help improve database performance:
   
   
   1. Separate the database and transaction / redo logs to different physical drives. This helps avoid contention on the disks because it increases the number of write operations that can be done at the same time.
   2. Avoid the use of statistics collection levels above level 2, unless troubleshooting a specific problem. The amount of data collected at the higher levels can cause both slow performance of the database and excessive growth.
   3. Avoid running other applications on the server which is hosting vCenter Server. This can appear to be a database problem. However, it can be a factor in slow performance of the database due to resource contention on the vCenter Server system.
   4. Do not use SQL Server Express. It is only designed to run a limited configuration and quickly hits the 4 GB database size limit. This causes a decrease in performance, which can then lead to failure of the service.

For more information on performance recommendations, see the VMware vCenter Server Performance and Best Practices document.
 
For more info refer this KB