MySQL Reference Manual :: 7.4.9 Drawbacks to Creating Many Tables in the Same Database

If you have many MyISAM or ISAM tables in a 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 making the table cache larger.