Turn on or off SharePoint Developer Dashboard

SharePoint developer dashboard can be turned on or off via Powershell script as follows
Make sure that "Usage and Health Data Collection" service application has started first under Manage Service Applications.

Add-PSSnapin "Microsoft.SharePoint.PowerShell"
$contentService = [Microsoft.SharePoint.Administration.SPWebService]::ContentService
$dashboard = $contentService.DeveloperDashboardSettings
$dashboard.DisplayLevel = "On"  # "Off" will turn off dashboard
$dashboard.Update()

After the dashboard is turned on, you will see a perfmon icon in the right upper corner of SharePoint site next to "SHARE", "FOLLOW", 'EDIT" menu items.

***In case the "Usage and Health Data Collection Proxy" has not started, use the following Powershell to start it.

$SAP = Get-SPServiceApplicationProxy | 
 where-object { $_.TypeName -eq "Usage and Health Data Collection Proxy" }