>>I am allowing a starts with, ends with, =, >=, <=, <>, empty, not empty filters.
>
>I'm not sure how helpful this will be, but you can usually avoid empty(), which is NOT optimizable.
>
>for instance:
>
>field={}
>field=' '
>field=0
>
>Could make a big difference.
>
>I've also found that the order of the statements in the WHERE clause can make a big difference. It seems that putting those that filter the most records first is best, but you might want to play around with the order a bit. VFP's reporting of opimization is not the end-all of optimizing your SQL.

All you have to do is create an index on Empty( < field > ) and then both the Empty and Not Empty will be optimized.