MySQL 8.0 Release Notes
MySQL 8.0 Source Code Documentation
If you have many MyISAM tables in the same
database directory, open, close, and create operations are
slow. If you execute SELECT
statements on many different tables, there is a little
overhead when the table cache is full, because for every table
that has to be opened, another must be closed. You can reduce
this overhead by increasing the number of entries permitted in
the table cache.