How do I allow mysqld to use more than 24.9% of my cpu?

Because you use a OLD version of MySQL that don't scale well with multiple cores.

Try to upgrade :)

This seems an obvious effect of updating a single MyISAM table. The whole table is locked during writes.

Either use more tables, or switch those with heavy writes to InnoDB. Since the table engine choice is per table, not per database, you can choose what is best for each one.

Remember, MyISAM is fast for mostly-read tables, InnoDB is best for almost everything else.

Also, if you have a single table with so many writes, consider changing your architecture. check queue managers (to send those updates to a non-interactive task), or append-only engines (fast for logs or archives, if that's what you're doing), or a different kind of server for tight jobs (like an in-memory DB if you're doing a lot of temporary processing before writing final results to more persistent storage)