accessing a linked dbase IV file is slow...

I have an access 2000 app that uses a linked table to an older application
built in dbIV.

I have a combo box whose record source property is set to an SQL select
clause from this 'linked' table. The combo box allows the user to scroll
through the records or they can simply begin entering a portion of the field
and it will filter down to what they want.

The users are complaining that this process of case# selection is very very
slow - and indeed it is. There are many records in this linked dbase IV
table to boot.

Can anyone think of anything I could possibly do to speed this up? This
particular dbase file I am linking to is being updated on a regular basis
throughout the day by an old legacy dbaseIV application and as such I really
can't create a duplicate of it to use in my app (which would really speed it
up for sure.)

Does anyone have any ideas or are they going to have to just live with it
until the old legacy dbase app is converted (it is scheduled to be - but not
for another year anyway...)

Thanks, Brad