Enable Developer Dashboard

SharePoint 2010 has many new features, among them, the Developer Dashboard. This is a dashboard for analyzing the execution and performance of the current page. The information provided will execution time of queries and events to more general information as the user logged on or page state.

By default this feature is not enabled. You can choose between four methods to activate it: a STSADM command, a PowerShell script, a .NET code or the Developer Dashboard configuration feature.
There are three levels of activation:

• Off: Disabled, this is the default value.
• On: The feature is available on all pages
• OnDemand: An icon is displayed on the pages to enable or disable the feature.

The state OnDemand is, in my opinion, the most practical because you can do this only on the pages under development.

This operation requires that you be administrator of the SharePoint farm.


STSADM

    stsadm -o setproperty -pn developer-dashboard -pv on
    stsadm -o setproperty -pn developer-dashboard -pv off
    stsadm -o setproperty -pn developer-dashboard -pv OnDemand (recommended)

PowerShell

$svc=[Microsoft.SharePoint.Administration.SPWebService]::ContentService
$ddsetting=$svc.DeveloperDashboardSettings
$ddsetting.DisplayLevel=[Microsoft.SharePoint.Administration.SPDeveloperDashboardLevel]::OnDemand
$ddsetting.Update()

Refer:

http://en.hugon.ws/articles/sharepoint-2010/using-the-developer-dashboard/
http://blogs.technet.com/b/patrick_heyde/archive/2009/11/16/sharepoint-2010-enable-using-developer-dashboard.aspx
http://blogs.technet.com/b/patrick_heyde/archive/2009/11/16/sharepoint-2010-enable-using-developer-dashboard.aspx