The distribution of response times and distribution of unread notifications dashboards miscount the time that it takes a notification to be read because they (understandably) treat the difference between two MediaWiki timestamps as the time difference in seconds. Sadly, that isn't true because MediaWiki timestamps are just integers (techically, BINARY strings) that look like timestamps.
I'm working on a fix for this; we'll have to rerun the queries for past periods as well.