Currently, 2DSphere points are indexed by default in cells of size ~500x500 meters. During $near queries, because points are indexed too coarsely, less can be filtered out during index scans. Indexing at finest level should give minor improvements in performance.
Furthermore, with points indexed at finest level, queries on dense data (e.g.
SERVER-18724) can be improved by modifying the query finest covering level to be finer.