Performance on same table vs 1:1 relationship in MySQL
1:1 is usually a 'wrong' design.

One exception is when some column (such as "Likes" or "click count") is very frequently incremented.  Putting that in a different table offloads the main table, and cuts back on the incrementation from interfering with other operations (and vice versa).

When does the recommendation kick in?  You would have to test it for yourself.

However, one thing can be said with a fair amount of certainty -- you really should switch from MyISAM to InnoDB.  UPDATE locks the entire table for MyISAM.  For Innodb it only locks the rows to be modified.  This can make a big difference when updating lots of different rows, or other high-volume activity.

Conversion tips .