Today I did some inspections of slow queries on my MySQL servers and I found this sweet query from a s9y plugin:
SELECT count(sessID) FROM s9y_visitors WHERE '32char md5sum' = sessID GROUP BY sessID;
This query seems to be performed with every site impression of your blog (when the plugin is installed). Not only that e. g. the GROUP BY makes no sense here (besides of slowing down the query), this query can take very very long time in execution (2 to 10 seconds) if you have lots of site impressions (the s9y_visitors table of the blog, where I found this, currently holds over 500.000 rows). For sure there is no rotation or expiration for this table. Dear developer of this plugin, please try again and build it more performance sparing!