When comparing Google Pagespeed Insights Online vs GTMetrix, the Slant community recommends Google Pagespeed Insights Online for most people. In the question“What are the best website speed profilers?” Google Pagespeed Insights Online is ranked 2nd while GTMetrix is ranked 3rd. The most important reason people chose Google Pagespeed Insights Online is:
Considering the fact that a large fraction of people nowadays visit websites from mobile devices, it is very important that your site performs well on mobile browsers too. To help you with increasing your site's performance in the mobile arena, Google Page-speed has you covered, it'll test your site with respect to mobile platforms and provide you a detailed report of your current score and what you can do to improve it.
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Pros
Pro Provides mobile pagespeed score as well
Considering the fact that a large fraction of people nowadays visit websites from mobile devices, it is very important that your site performs well on mobile browsers too. To help you with increasing your site's performance in the mobile arena, Google Page-speed has you covered, it'll test your site with respect to mobile platforms and provide you a detailed report of your current score and what you can do to improve it.
Pro Free
Pro Provides a very detailed report of its findings
Once you test your website using Google pagespeed online, it not only gives you a score of how fast your website is, but also what you can do to improve it. These optimization tips can range from minifying assets and optimizing images to leveraging browser caching. Following these you could improve, not only your pagespeed score but also your rank in Google search results
Pro Provides a very detailed analysis report
Like all of its competitors GTMetrix also does provide an extremely detailed report of its findings, which include a really useful waterfall feature and the famous and extremely informative YSlow website speed grading as well.
Pro Compare results with other websites
GTMetrix allows you to compare your site's speed reports with those of upto 4 other websites.
Pro Supports location based testing
GTMetrix.com have their servers in a number of countries, these give you the freedom to profile your website from the location where most of your traffic originates from
Cons
Con No option for location of test
Even though Google's Pagespeed Insights does quite an amazing job as a speed profiler, currently it doesn't provide you with the option of analyzing your website from a specific location.
Con Limited actionable data
Provides unhelpful instructions like "eliminate blocking JS", well duh, but what JS is blocking? Without a waterfall/HAR like the other's GPS is seriously lacking in usefulness.