Welcome to the Stack Overflow test domain!
So what is it that we do here?

We are always seeking ways to improve performance and usability for our users. Most our pages render in less than 100ms, the core pages like a question page renders in less than 40ms on average. However, that doesn't help users who are far away from our data center waiting considerably longer to get the page we just rendered.

We can do more to improve load time for those users, hopefully by quite a bit. This domain helps us evaluate changes in DNS, geolocation, SSL termination, load balancing, and routing so that we can make all Stack Exchange sites faster all over the world. You may have arrived here from looking at our iframe, welcome! We are not adding this extra hit to every request, but less than 1% of the requests to our network. The intent is to keep ay eye on performance and evaluate infrastructure changes and the impact they have.

Browser Timing Info
Checking browser timings
This is a breakdown of the time that went into loading this page:
Breakdown Start Time Name

For a detailed description of each timing, you can see the performancetiming interface at W3C.

Unfortunately your browser does not support the JavaScript Navigation Timing APIs, so we cannot gather any useful information about this page load.
Full Disclosure
Here is exactly what's sent to our servers so we can track page load performance around the world:
Your timings were successfully submitted, thanks!
Uh oh, we had an issue submitting your timings - we have logged the exception and will be taking a look.