I would leave it at 10 sec for now, but open up your slowquery log and look at those queries. Run them from mysql command line using the EXPLAIN command. It will provide you the output of some of those queries telling you how well the joins are and which subquery (if any) is causing the major delay. Again, this is still an assumption that disk tables are part of your issue.
key_buffer = 128M
max_connections = 500
max_user_connections = 500
are you using the innodb engine for anything?
wordpress and the php settings are beyond me...