If the lists in fields in the Filter By Form window take too long to display or they aren't displaying values from the underlying table, you can change Filter By Form performance for all tables, queries, and forms, or change performance for a specific form. In either case, you can prevent the lists from displaying the underlying table's field values, display field values on the list for certain types of indexed or nonindexed fields only, or change the record limit that determines if the list displays a field's values.
Optimize Filter By Form performance for all tables, queries, and forms in the database
Optimize Filter By Form performance for a specific form