One of the biggest reasons that large-scale WordPress sites slow down is because their database uses the MyISAM type which isn’t optimized for speed. Convert to InnoDB for better performance.

• InnoDB has row-level locking. MyISAM only has full table-level locking.
• InnoDB has what is called referential integrity which involves supporting foreign keys (RDBMS) and relationship constraints, MyISAM does not (DMBS).
• InnoDB supports transactions, which means you can commit and roll back. MyISAM does not.
• InnoDB is more reliable as it uses transactional logs for auto recovery. MyISAM does not.

Conversion is explained in the document, but here are quick steps:

1. Logon to phpMyAdmin and select the database to see the tables
2. Export the database as a backup
3. Sort the tables by database type
4. Copy & Paste (as text) all rows that are MyIsam into the first column of Excel
5. Make sure that the table names are in the first column and remove everything else
6. Copy the below formula in the second column of all rows
7. Copy the second column in the SQL box of phpMyAdmin and run
8. Repeat step 3 and 4 as there might be an additional table now.
9. Run a repair + optimize

="ALTER TABLE "&A1& " ENGINE=InnoDB;"Code language: JavaScript (javascript)

Source: https://kinsta.com/knowledgebase/convert-myisam-to-innodb/