Web performance optimization (WPO) is the science of making webpages load faster, in order to provide a better experience to the user.  It follows, then, that the best metric for assessing performance is to determine ? you guessed it! ? how long it takes for a page to load.  For years, metrics that determine page load time, such as Time to Interact, were the basis of WPO.  Faster site = better experience.  Simple!

Lately, though, some performance experts have been questioning that formula ? or rather, the exclusiveness of that formula.Here’s why.

In the past decade, website content has exploded in complexity and size.  When sites were simpler, anyone could achieve ultra-fast performance if they paid for a top-shelf CDN like Akamai.  But with today?s heavy, complex page content, the bottleneck has shifted to the front-end, and site owners often reach a roadblock of performance.  They find they simply can?t make the page any faster without cutting down on valuable content.  Everyone wants Google-esque performance, but pages with hundreds of images cannot load that fast — no matter how much time and money is poured into WPO.A New Key Metric in WPO

Facing difficulties in optimizing today?s websites, developers and WPO experts sought other avenues for improving user experience.  What has emerged is a new emphasis on improving the page load process, not just the end load time.  This involves new metrics and new ways of thinking about WPO.

For an example of what this is all about, look at the screen shots below.  Put yourself in the shoes of a new visitor, coming to the page from a search engine.  Which feels better to you?

These two sites have the same overall load time, but the bottom site displays important content much faster than the top.  It?s an example of just how different ? and how much better ? a page load experience can be.  Users are less likely to bounce away or become frustrated with sites that show progress earlier, rather than remaining stuck on the ?white screen of death.?

The metric that indicates how fast a page shows visible content is known as Time to Start Render.This metric has emerged as a key metric in optimizing page load process, and a key metric in WPO overall. In the example above, the bottom page beats the top in Time to Start Render by over 1 second.

WPO used to be all about improving page load time. Now developers must take into account the load process as well as the total load time: Time to Start Render and Time to Interact.

How To Optimize Start Render time

If Time to Start Render is new to you, you’re in luck: it’s really easy to find out how your site performs for this metric.  On Websitetest.com (or your go-to testing service) test your site across different variables like browser, device, and location. Time to Start Render is one of several metrics that are collected by default.  If you want to learn more about testing and assessing performance metrics in general, check out our eBook on 17 Performance Metrics You Should Know About.

The median for Time to Start Render across the web is 2.5 seconds.  Shoot for better.  The top 10% of sites on the web start render in less than 900 milliseconds — fast enough that the visitor doesn’t have time to think about the fact that he or she is waiting to see content.  That should be the goal.

How to Optimize: Content or Delivery?

For content, the most important way to optimize start render time is to reorder the asset download sequence on your page. This mostly means putting CSS at the top of the sequence and JavaScript files at the bottom. That way, content that is dependent on CSS for positioning on the page can be displayed as soon as it?s delivered, and no time is wasted on parsing JavaScripts that may be in the way.

Content optimizations that help Time to Start Render:

• CSS at top
• JavaScript at bottom
• Data URIs to inline content into HTML
• Post-load components

(Read more about optimizing order in our free eBook: How to Optimize Order of Execution)

The other way to improve Time to Start Render is to use a CDN.  This positions content closer to the visitor, and enables faster response.  Unfortunately, the only way sites can improve on their Start Render time with a standard CDN is if the pages are made up of static (non-changing) content any dynamic element (like personalization) will prevent the HTML file from being cached.

To solve this, Yottaa has invented InstantON, a feature that extends this capability to dynamic pages for our Yottaa Site Optimizer customers.  InstantON cuts Start Render times dramatically for any site, regardless of content complexity and type.

What’s the future of WPO?

Despite the changes in emphasis on WPO metrics, we don?t see overall page load diminishing in importance.  Users still want to access pages as quickly as possible ? and they?re getting more impatient all the time.  There?s no replacing a fast Time to Interact as the ultimate victory in web performance.

But until every page loads in under 1 second, Time to Start Render will remain one of the most important metrics to optimize.  And since pages are still getting slower on average ? not faster ? that time could be far off.