Photo: tobias.munich/Flickr

Anyone who’s ever tried to optimize a website has faced the very basic question — how long does your site take to load?

The answer seems like it would be easy to discover: Load your site in a page speed crawler like WebPagetest and soon you’ll have your numbers. But that’s just it; you won’t have a number, you have numbers and figuring out which numbers to listen to is trickier than you might think.

Strangeloop’s Joshua Bixby recently tackled the performance metric question in a blog post titled a Non-Geeky Guide to Understanding Performance Measurement Terms. The whole article is well worth reading, but perhaps the best advice is to make a video of the page load. “If you want to get a ground-zero look at your site’s performance,” writes Bixby, “capturing videos and filmstrip views of your pages’ load times are one of the best ways to go.”

The filmstrip view Bixby refers to is part of the WebPagetest results and shows what the visitor sees in a progressive series of page captures. To create a filmstrip or video test of your website, head over to WebPagetest and select the “visual comparison” tab.

Some common performance mistakes Bixby cautions against include using “response time” and “load time” interchangeably and “not realizing that ‘response time’ can mean any number of completely different things.”

To help those unfamiliar with the nuances of loading metrics, Bixby then breaks down and defines all the terms, including what exactly is meant by “start render” or “time to first byte,” as well as some caveats to bear in mind when going over the numbers for your website.

While Bixby’s post can be extremely helpful, especially to those who are just starting out in the often confusing world of website optimization, bear in mind that testing sites like WebPagetest are no substitute for real-world tests. “As a matter of due course, you always need to gather large batches of data and rely on median numbers,” writes Bixby, “but you also need to periodically get under the hood and take a real-world look at how your pages behave for real users.”