If you are running Kingston, take a look at replacing the Business Rule with a GlideFunction field.

https://docs.servicenow.com/bundle/kingston-application-development/page/build/platform-functions/ta...

This moves the calculation from the application side to the database.   I haven't tested this, but it should give a performance boost.   In addition, GlideFunctions can be used in a query.   If someone queried the u_incident_age field in this example, they would get incorrect results (since what is stored in the database is what was calculated on the last read instead of what is there right now).

If you are pre-Kingston, you may want to look at doing this as a Calculated Field instead of the BR.   Functionally, I believe it should be the same, but the logic will be more self-contained.