Async I/O in Flink handles non-blocking external calls (e.g., databases, APIs) using AsyncFunction, improving throughput by avoiding I/O bottlenecks.

Multi-slot execution manages task parallelism by allowing multiple tasks to share a TaskManager’s slots, optimizing resource usage.

Key Difference: Async I/O improves external call efficiency, while multi-slot execution optimizes resource allocation in Flink’s cluster.