In most aspects, indexed fields mean better performance. In some aspects, it can mean much slower performance.
Searches and sorts are many times faster when they are done only on fields that are indexed.
But actions that require the addition, deletion or change of large numbers of records in a table will be much slower if the fields in that table are indexed. So importing large blocks of records, deleting large blocks, running a Replace Field Contents or looping update script to modify a large block of records will be much slower.