The Slow/Fast speed graph that used to appear for me within Labs / Site performance has disappeared for nearly all my sites?! For the few sites where the graph still shows, there is so little data (just a few days in a past month - different period for each site) for it to be completely useless. It used to show a lot more...?!
Yeah, most I've queried this with can still see the graph for their sites. However, it has been suggested the graph has disappeared for some low traffic sites - although I don't see how WMT can possibly know how much traffic the site is getting!? (Unless Google Analytics is also installed for that site!?)
Besides I don't consider all the sites in question 'low traffic'.