According to the Site Performance feature in Google Webmaster Tools, your pages load reeeeealy slow but, other external tools or monitoring services tell a different story.
What should you believe?
First, it's important to understand the differences between these tools, the data they capture and how it's measured.
Page Speed evaluates the performance of a specific web page and individual elements in the browser. As a result, this type of testing may not accurately reflect latency experienced by users. Page Speed is for testing and improving speed for individual pages.
Tools like webpagetest.org and monitoring services often test latency for a specific URL at various times of day and locations around the world. As a result, these kinds of tests may not reflect latency as perceived by users in the region the site targets.
Google Webmaster Tools Site Performance data is collected from actual Google Toolbar users in the same geographic region as the target audience of the site. This data can be measured in several ways. One being, time between when the user clicks on a link "until just before that document’s body.onload() handler is called." If for example, if a user clicks on a link, is then redirected and then redirected again, that delay should be recorded and reflected in Google Webmaster Tools Site Performance data. These are the kinds of delays that impact users and Googlebot and that are totally missing from other tools including analytics.
Speed doesn't currently have a major impact on rankings but, slow pages deter users and hamper crawl efficiency. Crawl efficiency can be a major factor for pages with lower PageRank because " the number of pages Google crawls is roughly proportional to PageRank".