The Developer Dashboard is a part of the SharePoint On-Premise versions. I think this feature is one of the most forgotten and underrated. Especially in large farm deployments it offers a lot of functionality to identify bugs, errors, performance issues and bottlenecks.

How to enable the Developer Dashboard?
Prerequisites
The developer dashboard is dependent on the “Usage and Health Data Collection”. Be sure to have it up and running.
Give me the code!
The following lines of powershell code enable the Developer Dashboard.
$contentService = [Microsoft.SharePoint.Administration.SPWebService]::ContentService $developerDashboardSettings = $contentService.DeveloperDashboardSettings $developerDashboardSettings.DisplayLevel = [Microsoft.SharePoint.Administration.SPDeveloperDashboardLevel]::On $developerDashboardSettings.Update()
To deactivate the dashboard change the value of the DisplayLevel to Off.
To launch the Developer Dashboard window click on the respective icon in the upper right of your SharePoint site (marked with yellow circle below). It’s available after a page refresh.

What’s new with SharePoint Server 2016?
RUM – Real User Metrics
RUM stands for Real User Metrics and it’s an aspect of SharePoint Online telemetry.
Though this won’t help on-premises, it’s good to see and know that SharePoint 2016 uses a lot of the code that’s run by Microsoft in SharePoint online.
Can I customize the Developer Dashboard?
Stefan Goßner has a great write-up about customizing the SharePoint 2013 Developer Dashboard. All you need to do, is making a JavaScript file available and insert it into the dashboard via powershell.
$developerDashboardSettings.userscripts.Add("path to your script")
How does it compare to the ULSViewer?
In my opinion the ULSViewer and the Developer Dashboard go hand in hand.
With the ULSViewer you get a unfiltered view of your SharePoint environment and everything happening there. Sometimes it is hard to find the right spot in the logs.
With the Developer Dashboard you’ve got a tool in your hand, that shows only the most important data for a certain user request. The Developer Dashboard has even more additional information about SQL Server, Animations and the MDS.