The feature of Swagger that allows developers to design, build, and document APIs directly in the browser is the "Swagger Editor." It provides a browser-based interface for designing and documenting APIs, making it easier for developers to create and visualize API specifications. Swagger UI, on the other hand, is used for visualizing and interacting with already-documented APIs.

OpenID Connect extends OAuth 2.0 by adding JSON Web Tokens (JWT) to enable Clients to verify the identity of the End-User based on the authentication performed by an Authorization Server. JWTs are a compact, self-contained format for representing claims between two parties, making them suitable for securely transmitting identity and other information between the Client, Authorization Server, and Resource Server.

To optimize web application performance, utilizing caching for frequently requested data through the Web API is crucial. This approach reduces the need for repeated API calls, improving responsiveness and load times. Synchronous API calls can block the application, increasing latency. Increasing payload size might slow down requests, and adding unnecessary authentication layers can have the opposite effect.

In Node.js and Express, the res.json() method is used to send a JSON response to the client. It serializes a JavaScript object into JSON format and sends it as the HTTP response, allowing for structured data interchange between the server and client.

In a microservices architecture, an API Gateway can help in load balancing, thereby simplifying the client-side communication. Load balancing ensures that client requests are distributed evenly across the microservices instances, optimizing resource utilization and improving system reliability. This enhances the client's experience and overall system performance.

Caching mechanisms play a crucial role in optimizing API performance and scalability. They reduce the load on the server by storing frequently requested data. This results in faster response times and better scalability, as the server can handle more requests efficiently. However, caching should be managed carefully to avoid data security issues and ensure that users receive up-to-date information.

To ensure smooth operation and performance in the face of heavy traffic, an API Gateway can be configured with rate limiting and caching strategies. These strategies can help manage traffic spikes, prevent overloading the backend services, and improve response times. Increasing the number of instances can help with load distribution. Disabling caching may not be a good strategy for performance. Removing the API Gateway is not a recommended approach.

OpenID Connect handles user authentication and authorization securely by using identity tokens for authentication and OAuth tokens for authorization. This separation of concerns enhances security. Identity tokens contain user information, while OAuth tokens grant access permissions. The combination of both ensures a secure and standardized authentication and authorization process.

Partner APIs play a crucial role in a business ecosystem by enabling collaboration and data exchange with external organizations, such as partners, suppliers, or customers. They allow businesses to extend their services and reach beyond their internal boundaries, fostering strategic partnerships and creating new revenue opportunities.

Implementing Role-Based Access Control (RBAC) in APIs offers advantages such as fine-grained access control, which allows for precise permissions management. This enhances security by limiting access to sensitive data and functionality based on user roles. It does not necessarily simplify development but ensures secure access.