File collection field indexing

We are using the gridfs bucket[docs] to store our documents.
I added one field 'parentId' in docs.files collection.

I'm querying on this field['parentId'] to get a file from the gridfs bucket.

I'm facing the slowness while retrival of the file.

Can I index the 'parentId' field in the docs.files collection to improve the performance?

Note : This gridfs bucket is sharded.