Building a multiplayer game using JavaScript can be complex, but leveraging tools like PubNub and Phaser API simplifies the process by managing the challenging back-end infrastructure and enhancing game performance through efficient design patterns. This approach focuses on player movement by using interpolation to estimate positions based on input events, which reduces the number of messages sent and minimizes performance lag and costs. The key technique involves only sending messages when keys are pressed or released, rather than continuously updating coordinates, and utilizes PubNub's swift message delivery to maintain synchronicity between users' screens. Key input handling is demonstrated through code snippets, showing how messages are published with player-specific information such as UUID and position, which are crucial for accurate and real-time player movement representation across clients.