The time required to delete individual rows in a MyISAM table is exactly proportional to the number of indexes. To delete rows more quickly, you can increase the size of the key cache by increasing the key_buffer_size system variable. See Section 7.11.2, “Tuning Server Parameters”.

To delete all rows from a MyISAM table, TRUNCATE TABLE tbl_name is faster than than DELETE FROM tbl_name. Truncate operations are not transaction-safe; an error occurs when attempting one in the course of an active transaction or active table lock. See Section 12.1.27, “TRUNCATE TABLE Syntax”.