It depends. We feel that the use cases for each tool can be broken down into the following categories:

Quick Checks

GTmetrix, Pingdom Tools, and PageSpeed Insights

Consistency/Historical Tracking

GTmetrix and WebPagetest

In-depth Analysis

GTmetrix and WebPagetest

SEO Check

PageSpeed Insights, GTmetrix, and Pingdom Tools

Mobile Devices

WebPagetest, GTmetrix, and PageSpeed Insights

Location Dependent

WebPagetest, GTmetrix, and Pingdom Tools

Advanced Options

WebPagetest and GTmetrix

Pingdom defaults to Onload Time only

This is why you may see Pingdom results loading faster than GTmetrix and/or WebPagetest.

Fully Loaded Time

The point after the Onload event fires and there has been no network activity for 2 seconds. This ensures more consistency with tests.

Possible issues with using this event: This event fires only when a page completely stops loading content, including ads and below the fold elements. Your site might have loaded fast above the fold and be usable, however since the analysis is now waiting for the entire site to stop loading data, your reported Page Load Time might be longer.

Onload Time

When the processing of the page is complete and all the resources on the page (images, CSS, etc.) have finished downloading. The browser will trigger window.onload when this occurs.

Issues with using this event: Some elements of the page load may not make it in before this event fires – like JavaScript based image carousels – causing inconsistent page load times and inaccurate screenshots. It may also report faster page load times than actual.

Onload time may misrepresent the true load time

Due to network variances or the way the page was designed to load (eg. asynchronous loading), resources loaded after the browser triggered window.Onload may not make it into reports.

The result is a report that indicates the page loaded faster than it actually did. This means that the while the tool received a “page finished loading” trigger at onload, the actual user is still experiencing resources being downloaded.

Our preferred testing platform, GTmetrix now uses fully loaded time, which waits for 2 seconds of network inactivity before stopping the test.

The Other Platforms

GTmetrix and WebPagetest allow you to switch between Fully Loaded and Onload Time.

Pingdom defaults to Onload Time only.

This is why you may see Pingdom results loading faster than GTmetrix and/or WebPagetest.

Recommendations will differ between tools

Each of these tools assess websites against their own set of recommendations.

Most of them stem from Google’s original open source PageSpeed library, and have likely been customized or modified.

GTmetrix in particular has modified PageSpeed and YSlow rules to assess websites based on what they feel are the most important metrics.

Don’t compare these tools equally and wonder why your scores differ from tool to tool

While they’re all likely based on the original, open-sourced Google PageSpeed library, some key differences should be pointed out.

After Google made general scoring algorithm/recommendation changes years ago, they have not updated the open-sourced library since.

The PageSpeed Insights online tool as a whole seems to contain a new set of rules (which isn’t open-source).

Don’t compare these tools equally and wonder why your scores differ from tool to tool; they all use different recommendation sets.

This is why you’ll see differing rules from each service, and why GTMextrix deemed it necessary to update their own PageSpeed rule set.