I did my own small research on that topic. I generated 500,000 records (randomly generated strings) in a collection, and tried a couple of queries with explain() statement.
Then I ensured a unique index, and tried few other queries again:
As you can see, after adding index the time consumption decreased from ~276ms to 0ms! So it seems like even if the index is unique, it affects (in a positive way) the find queries.