See report in : http://forum.piwik.org/read.php?2,70690
The query should have AND visit_last_action_time > ?
clause to make it use the INDEX and perform better.
I have this problem too. piwik has become unusable since the critical update from 1.0.
(In [3758]) Refs #1966 Spy should not keep doing requests when the screen is not showing the widget anymore
Piwik was very slow for me and unusable; disabling "Live!" made it usable again.
we must take a decision: either we disable Live! by default for all new installs, or we fix these performance issues
I fixed this issue, just and introduced a new bug tracked in: #2076