From time to time Google introduces new features in Webmaster Tools in order to enhance its’ services, similarly they remove any features that they think are not worth having.
Early this year in April 2012 Google announced that there will be a round of spring cleaning process within their Webmaster Tools and they will be removing some of the features from Webmaster tools as a part of its clean-up process. Site Performance was one of them and as of 1st of November 2012 it is no longer supported by Google Webmaster Tools.
When you go to check your Site Performance under Labs in Google Webmaster Tools you will see the following message.
According to Google this feature has been removed due to low usage.
Google also confirms that the page load speed still is an important ranking factor by stating the following:
There are many other tools available that can be used to check the site performance of your website, that inludes site speed in Analytics, PageSpeed Insights and others.