I have a problem with extreme slow database queries that make my site loading for more than a minute. I am not able to locate the source of that problem. I'm using devel to control the queries and I found out that mostly the functions saving or updating the cache or session tables are the cause. Here is one example of devel's output:
19622.34 1 _drupal_session_write P A UPDATE users SET access=:db_update_placeholder_0 WHERE (uid = :db_condition_placeholder_0) 10442.86 2 DrupalDatabaseCache::clear P A DELETE FROM cache_bootstrap WHERE (cid = :db_condition_placeholder_0)
I do not have an idea what I could do to find the cause of that problem.