This might sound a little tongue in cheek but I run a massive database site and due to time constraints was forced to do several days of heavy database maintenance during the day.
This makes the site more sluggish because the database is under a heavy load, pages loading a bit (lot) slower than normal, but overall it's not the normal snappy response when we're doing this type of maintenance work on the live site.
We used to do it late at night to avoid such sluggish site problems but it was unavoidable this time, had to happen during prime time.
Well, of all the amazing things to happen, AdSense CTR actually increased, along with revenues, during this maintenance cycle!
Just speculating but perhaps people got tired of waiting on the longer page loads and used AdSense as an escape route.
Earn more, bog down your server today!
Just kidding, somewhat, but I thought the visitor behavioral observation was interesting at a minimum because we've read reports from people that upgrade to faster servers and income suddenly dips so maybe too much site performance is bad for AdSense performance?