Looking into Google Webmaster Tools in the Site Performance section I see that
On average, pages in your site take 5.0 seconds to load.
Specifically it takes about 5 second to load during last month. However when I personally look pages on my site it rarery takes longer than 1 second to load a page.
How can I discover the reason why Google thinks that pages load slow? Does Google provide any useful info for that? Should I look into web-server logs, etc.? Should I tweak something in robots.txt to prevent indexing some pages or to change crawl speed?