Try benchmarking Kafka from 0 partitions to a few thousand partitions in 100 partition increments. The benchmark only needs to write to a single topic, using their provided producer perf tool while all other topics are inactive with zero data.

As the partitions increase there is a very noticeable drop in throughout that looks to be linear.

Kafka does not handle a large number of partitions well currently, large even being low thousands. It's easy to hit with just a few hundred topics.

Reading between the lines ehen Linkdin and Netflix advertise several clusters, i am predicting/guessing they shard the data.