1. Webpagetest.org allows you to run the test from servers all around the world. This can help with assessing issues such as latency. Additionally, if you are using a CDN, then static resources will arrive much faster from the “edge servers” when originating the test in other geographies. Thus, it gives a better picture of performance from your users’ perspective than running the analysis from your machine.

2. WPT has a Test History that allows you to see previous analysis runs and visualize them on a graph. It will show Timings, Visual Progress, and Bytes of transfer on the same graph with each line representing a different historical analysis run. So it has helped me see the improvements made from different performance tuning I’ve been trying out. It will even superimpose multiple waterfalls on top of each other, and the controls let you slide back and forth to dial-in the opacity of each. It’s pretty cool to play with and visually identify individual requests that are now faster or slower.

3. WPT has a calculation that is unique – Speed Index. Patrick Meenan spoke at Velocity (NY Oct 2013) about Speed Index being an attempt to measure the perception of users regarding the performance of a page. From their site:

“The Speed Index is the average time at which visible parts of the page are displayed. It is expressed in milliseconds and dependent on size of the view port.

The Speed Index metric was added to WebPagetest in April, 2012 and measures how quickly the page contents are visually populated (where lower numbers are better). It is particularly useful for comparing experiences of pages against each other (before/after optimizing, my site vs competitor, etc) and should be used in combination with the other metrics (load time, start render, etc) to better understand a site’s performance.”

Those are some of the main differences that I can identify. Would you like us to write a blog that compares WPT, YSlow, and PageSpeed?

Thanks for commenting!