There were some Google Webmaster Help thread with reports that the site performance reports in Google Webmaster Tools was not updating that often. Normally, it should update every other day or so but recently it has not been doing so.
A Google Webmaster Help thread has confirmed reports of the issue and that it should now be resolved. sreeram from Google said:
Yup, there was another temporary glitch. Things are back to normal now.
I checked several site performance reports and they all seem to be within two days or so old.
Forum discussion at Google Webmaster Help.