Now there’s a big emphasis on page load times, people are beginning to take more scrutiny about the elements they add to a webpage. I myself have been guilty of whether or not to add different tools to my page.

I finally figured I’d take a look at what impact Google Website Optimizer actually has on page load times and I was pleasantly surprised – particularly given I live in Australia where the internet is notoriously horrible.

First, some measurements

• 310.8ms average load time for siteopt.js (the script that the control snippet loads) from a 1.5mbit connection in Sydney
• 21ms average ping time to www.google-analytics.com (where siteopt.js is hosted) from my work connection in Melbourne
• 36ms average ping time to www.google-analytics.com from my home connection in Melbourne
• siteopt.js weighs in at around 2kb – plus or minus some kilobytes, depending on what you stick in your sections

What this means for your page load times

• Since it loads siteopt.js synchronously in the header, it does stop other elements from loading (see below)
• Calls to _utm.gif and ga.js add to the latency of your site (ever so slightly)
• It’s hosted on the same domain as www.google-analytics.com so you only make one DNS lookup
• It uses the same technology as Google Analytics, so if you’re using Google Analytics, the impact is even less
• It’s not a huge performance impact – probably much better than other tools in the market
  • I doubt other tools without a data center in Australia or any other countries with crappy internet will perform this well

siteopt.js loading on my home computer (a beefy Core-i7 on ADSL2+). Notice how other elements aren't loading while it's requested?

While there’s not a lot you can do to speed up siteopt.js, you may find some small (yet seemingly insignificant) speedups by using asynchronous GWO tracking scripts which are available now if you create new experiments. Unfortunately, the same cannot be done for the control script which needs to load synchronously. Otherwise you’ll see a page changing right before your eyes. In fact this has been confirmed by Googler, Avinash Kaushik. So I wouldn’t hold your breath waiting for one.

Thoughts

Overall, I don’t think it’s a major thing to worry about, however, if it’s important that pages load incredibly fast while testing, then it’s worth taking a look at this.