MySQL 5.7 Release Notes
The more complex your privilege setup, the more overhead applies
to all SQL statements. Simplifying the privileges established by
GRANT statements enables MySQL to
reduce permission-checking overhead when clients execute
statements. For example, if you do not grant any table-level or
column-level privileges, the server need not ever check the
contents of the tables_priv and
columns_priv tables. Similarly, if you place
no resource limits on any accounts, the server does not have to
perform resource counting. If you have a very high
statement-processing load, consider using a simplified grant
structure to reduce permission-checking overhead.