Mysql High cpu load
tmp_table_size = 10G
NO! If 13 users are running a very big query at the same time, that could fill up RAM! Suggest lowering that only 1% of RAM. Let's say only 500M.
So, how does that lead to high I/O? A naughty query will first fill up the 10G, then if it needs more room, it will write it to disk.
The easiest way to make such a query is to JOIN two large tables without an ON clause.
A general help (for CPU, and potentially I/O) is to improve the indexes on postmeta: http://mysql.rjweb.org/doc.php/index_cookbook_mysql#speeding_up_wp_postmeta