Changing the mode of the Developer DashBoard

By default, the Developer DashBoard is disabled. You can enable it by using Windows PowerShell.

Add-PSSnapin Microsoft.SharePoint.PowerShell -ErrorAction "SilentlyContinue"
$DevDashboardSettings = [Microsoft.SharePoint.Administration.SPWebService]::ContentService.DeveloperDashboardSettings
$DevDashboardSettings.DisplayLevel = 'On'
$DevDashboardSettings.Update()

It displays an icon in the upper-right corner that allows you open up the Developer DashBoard.