bench/docs: add S3 benchmark

[ness-david-dedu]

Results

• Kafka Connect S3 Sink: up to ~250k msg/s
• Kafka Connect S3 Source: up to ~73k msg/s
• Redpanda Connect (single): ~60k msg/s
• Redpanda Connect (multi): ~73k msg/s
• Bucket walk (sequential):
  • Small objects: ~560 msg/s
  • Large objects: ~190 msg/s (~195 MB/s)

---

Key Takeaways

• S3 is latency-bound without concurrency
• Batching (flush.size) is the primary driver for write throughput
• Kafka Connect scales via parallel S3 writers (~4× higher peak)
• Redpanda Connect is capped by shared output concurrency
• Results reflect LocalStack limits, not full S3 scalability

---