Combobox slow to drop down

[TD]

I have a combobox that is sometimes slow to drop down when clicked on.
If you select an item in the list that is near the top of the list then
the next time you click on the combobox it drops down fast for you to
make another selection. If you click on an item from the list that is
way down the list then the next time you click on the combobox it takes
several seconds for it to drop down so you can make another selection
The combobox is filled with approxiately 720 items; I cannot reduce
that number. Why is it so slow to drop down and is there anyway to
speed it up?

Thanks,
TD