Enable Developer Dashboard

$dash =[Microsoft.SharePoint.Administration.SPWebService]::ContentService.DeveloperDashboardSettings;

$dash.DisplayLevel = 'OnDemand';

$dash.TraceEnabled = $true;

$dash.Update() 

Disable/ Turn Off Developer Dashboard

$content = ([Microsoft.SharePoint.Administration.SPWebService]::ContentService)
$appsetting = $content.DeveloperDashboardSettings
$appsetting.DisplayLevel = [Microsoft.SharePoint.Administration.SPDeveloperDashboardLevel]::Off
$appsetting.Update()