GraphQL handles real-time data and subscriptions by using WebSockets to establish persistent connections. This allows the server to push updates to clients as soon as new data is available, enabling real-time interaction without the need for frequent polling, which is both inefficient and less responsive in comparison.

In GraphQL, Subscriptions allow clients to receive real-time updates when data they are interested in changes. Subscriptions enable a publish-subscribe model, enhancing the real-time capabilities of GraphQL.

Load testing is essential for APIs to ensure they can handle expected traffic loads without performance degradation. It helps identify bottlenecks, scalability issues, and performance limitations, ensuring that the API can deliver a seamless experience to users under varying loads.

OpenID Connect is built on top of OAuth and is used for user authentication. OAuth is a widely-used authorization protocol that allows third-party applications to access a user's resources without exposing their credentials. OpenID Connect extends OAuth to provide user authentication and allows applications to verify the identity of end-users.

Rate limiting errors in APIs can be identified and resolved during debugging by examining HTTP status codes and headers. When an API rate limit is exceeded, the server typically responds with specific status codes or headers that indicate the issue. Understanding these indicators is crucial for debugging and resolving rate limiting errors.

GraphQL offers benefits over traditional REST APIs, such as better caching and query optimization. GraphQL clients can request only the data they need, reducing over-fetching and under-fetching, which can enhance the efficiency of data retrieval.

One way to optimize API performance is to reduce the latency time for each request. Latency represents the delay in communication between the client and server. Reducing latency leads to faster response times, improving the overall performance of the API.

A "Public" API is typically exposed to external developers and allows them to extend the functionality of a platform or application. It's accessible to external parties for integration.

By implementing caching, an API can efficiently handle multiple requests without degrading performance. Caching involves storing and reusing frequently requested data, reducing the need for repeated processing and resource-intensive operations, thus enhancing API efficiency.

GraphQL effectively handles under-fetching and over-fetching of data by allowing clients to specify exactly what data they need, eliminating the need to make multiple requests for related data and minimizing unnecessary data transfer. This results in more efficient data retrieval.