Couchbase Server 5.0 and 5.5 introduced a variety of features that enhance its functionality for developers, including the ability to manipulate sub-documents, an Eventing Service for automatic server-side functions, and the use of ANSI JOIN syntax for queries. These versions also bring Full-Text Search capabilities directly into the database, reducing the need for third-party tools, and offer performance improvements through faster queries and aggregation pushdowns. Security is bolstered with Role-Based Access Control, X.509 certification, and field encryption, while the Reactive SDK supports performance optimization through reactive programming. Developers can fine-tune performance with SDK options that balance speed and data safety, and Response Time Observability provides insights into request performance using the OpenTracing format, helping to identify and address operations with poor performance.