Please notice your username is DELAYED. This is an internal username for mysqld to process any INSERT DELAYED statements, which only works on MyISAM, MEMORY, ARCHIVE, and BLACKHOLE tables.
This may be the right time to switch your WordPress databases from MyISAM to InnoDB. If you do switch to InnoDB, change all INSERT DELAYED statements to INSERT or the statements will generate errors.
You have several options
Option 1) Move each WP database into a separate DB server (Bare-Metal or Virtual)
You can configure each database to the needs of the particular WP database. Of course, there are budget considerations outside the scope of this question.
Option 2) Create several instances of MySQL on the same DB server, one for each WP database
You can configure each database to the needs of the particular WP database, but datadir for each WP database should reside on a separate disk volume.
Option 3) Scale up the DB server
Add lots of more RAM and uses a faster disk volume. Then, configure the MySQL instance to utitlize as much RAM for MyISAM as possible. Keep in mind, MyISAM caching is only for Index Pages and not Data.
Option 4) Combination of Options 2 and 3
Option 5) Configuration of delay options
delayed_insert_limit: after performing this many INSERT DELAYED statements, pending SELECTs statements is given priority to run before allowing new INSERT DELAYED statements.
delayed_insert_timeout: seconds handeler thread waits before timing out.
delayed_queue_size : per-table limit of the number of rows. If queue is full, any new INSERT DELAYED statements will pause until the queue has room.
Options 6) My original proposal. Convert everything to InnoDB !!! You could cache both Data and Index pages in the InnoDB buffer pool.