Web app performance: Update record or fields?

I have a form with almost thirty fields in a MySQL database, using PHP to query and update etc. using stored procedures.

It occurred to me that saving the entire record every time it gets updated (when the user clicks save) might be significantly less performant than saving each field as it gets updated (i.e. when the focus leaves the input and the value has changed).

Obviously there's more connections involved in the latter approach but I'm thinking it might be better overall and wondering if anyone more experienced has any input?

Note that most of the fields are integers and dates, there are three varchar(2000) fields.

Thanks

Mark