$this->db->having() makes my query a bit slow

[eluser]johnwbaxter[/eluser]
I've got a little app that queries a database. Through a series of forms it allows the user to build up the query which is simply a select query with "where" for each section.

The next stage is it allows you to choose extra fields where there are no null values.
So i have done $this->db->having('Field Is Not Null') (could be up to 3 of these.

Anyway, the normal query (+ loading of page which is not much load) takes 0.0819 seconds then when i add the "having" part the query shoots up to 0.4237 seconds.

Is there a better/quicker way of adding a Not Null specification to the query?

Any advice would be very much appreciated.