Sounds like your field is not only a calculation but one that is not indexed. Some calculations cannot be indexed because they refer to a global field or a field in a related table. If this is not the case, you can open the field's calculation and click storage options to turn indexing on.
Performing a find on an indexed calculation field should go much faster for you.
Another trick that is sometimes useful is to perform your find in two stages.
- Enter find mode and specify criteria only in local fields that are indexed
- perform the find
- Return to find mode and specify criteria in related fields and unindexed fields.
- Select Constrain found set to reduce your found set to just those records that match this second criteria.