Speed Up Index Creation in MySQL
Use InnoDB because it supports Fast Index Creation.*

If you have plenty of RAM, configure a tmpfs partition and point your MySQL tmpdir to that location.



* There are many other reasons besides that to stop using MyISAM. MyISAM is known for its table-level locking, tendency to corrupt data, and lack of support for ACID and referential integrity. It's also slower than InnoDB under most workloads.