no idea if the situation would be similar, but...

I recently ran into some problems with an app that used parameterized queries generated by some OO middleware. This middleware was pretty flexible, but it was creating the parameters as nvarchars while all the fields we use were just varchars.

SQL appeared to be creating/holding a bunch of locks while it converted every value in the table or index to unicode, THEN do the comparison. So basically it was having to scan through & process every row vs. doing a simple index seek.

I wonder if you could feed some of those dropdowns via. separate views or indexed views.