A Primer in Web Performance

Performance matters. A slow site has serious effects on revenue, because both users and search engines dislike slowness.

The performance of your website directly impacts revenue per visitor:

  • According to Google, a increase in load time from 1 second to 5 seconds causes a 90% increase in bounce rate for mobile users.
  • Google also found that a mere 400 ms (0.4 seconds) delay caused a persistent drop in daily searches per user of 0.6%.
  • Walmart saw up to 2% increase in their conversion rate for every second of reduced load time, and up to 1% revenue increase for every 100 ms (0.1 seconds) reduction.
  • Aberdeen Group found that a one second delay decreased conversions by 7%.
  • Microsoft studied performance for their Bing search engine and found that a 2 second delay reduced revenue by 4.3%, and reduced user satisfaction by 3.8%.

And has indirect effects on the amount of visitors coming to your site:

What causes bad performance?

A web page consists of a document (HTML code) and external resources (JavaScript, CSS and image files). To display a page, the browser needs both.

Displaying a web page involves five steps:

  1. The browser connects to the server and asks for the document.
  2. The server generates the page and sends it to the browser.
  3. The browser analyses the page to see which external resources it needs. It asks the server for those external resources.
  4. The server sends the external resources to the browser.
  5. The browser combines the document and the external resources to display the web page.

The reason for slowness can usually be found in step 2, or in steps 3 to 5. Therefore, the causes for slowness can be divided into two categories:

  1. A slow server.
  2. A page with too many external resources.

Because step 2 must happen before anything else does, that's the first thing we should look at. Continue to investigating server slowness.