>I tested it without the combo box and all works fine. The form loads ok and does not seem transparent. Do you have any suggestions about how to speed up the combo box?

I agree with Nansy Folsom (messages of 24/09/1998).
I would try the following form layout:

=================================
! ------------ !
! Grid1 : Grid2 : !
! : : !
! ------------ !
!=================================

Grid1 being your already existing grid,
Grid2 is what I will use instead of the combo, Grid2 will display the same information as the late combobox.
When the form starts grid2's visible and enabled properties are set to false.
When I click on an appropriate column's control of grid1 grid2's visible and enabled properties are set to true, when grid2 loses focus these properties are again set to false.
I am not 100% sure because I have not tryed it on my forms. I hope it play out well.