I have done the same modification in my database server before I read this forum. It works properly now.

Since Artio SEF create a lot records for lookup the new direction URL, the database always need to access the jos_redirection table. Without the above NEW index, the database server need to do a "table scan" to search the information. "table scan" is always heavy loading task. It is a simple database design problem. All you need is to create a simple index only.

BTW, this SEF component works very well in my website.