Multiple listbox questions

I have a sales/cost/profitability report that with lots of help from posters
here I have managed to create a form to filter it by several criteria and its
all working well, I'm just tweaking it some now and have a few questions:

First I have 2 listboxes 1 for customers (we currently have 346 listed), 1
for lots (we currently have 663 listed). On both of them (both multiselect)
the slider does not work quiet right. On first load if you drag the slider
all the way to the bottom, it only goes down about half the list, release the
mouse button and the slider appears mid way down instead of at bottom where
you left it, then you can drag it the rest of the way, I assume its because
we have so many items in both boxes. Its definitely not a big deal but would
be handy to resolve this.

Second I have a start date and end date filter by drop down boxes and then
check boxes for each of our plants to filter by. I have added after update
events to update our 2 long listboxes described above with the following code
in both:

SELECT sales.Customer FROM sales WHERE (((sales!Plant) In
([plant1]*-1,[plant2]*-2,[plant3]*-3,[plant5]*-5,[plant81]*-81)) And
((sales![Month End]) Between [StartDate] And [EndDate])) GROUP BY
sales.Customer ORDER BY sales.Customer;

After adding the above loading the form slowed considerably, again I realize
I'm dealing with a lot of info, but am I doing it in an ineffecient way?

Finally I considered having 1 listbox update the options on the other
listbox but fear that would make things extremely slow, and possibly even
going both ways, as you update 1 list box the other one updates its available
options if thats even possible. Would that be unadvisable?

Thanks