Complex and heterogeneous power system automation environment requires a novel approach for enabling interoperability and interchangeability of multi-vendor devices and applications. OPC Unified Architecture (OPC UA) represents the state-of-the-art middleware technology for industrial automation domain. Moreover, the OPC UA is one of the potential middleware mappings for the next edition of the IEC 61850 standard which has been recognized as a one of the most prominent Smart Grid integration technologies. However, utilizing OPC UA in the energy automation requires improved communication features and optimized performance capabilities which are not directly applicable according to the current OPC UA definition. This paper proposes integration of several performance optimization techniques into the OPC UA middleware implementations. These techniques include support for asynchronous communication and utilization of the alternative message encoding mechanisms. The proposed OPC UA middleware has been applied in the IEC 61850-OPC UA mapping use case. Additionally, the prototype of the proposed solution has been developed and analyzed in the middleware simulation environment.