Database latency vs throughput
Your intuition is close, but disregards concurrency.

You can have medium/high latency, high throughput, massively concurrent systems - any individual request may take longer on Cassandra than on some other systems, but you can run far more requests in parallel, allowing total throughput to be higher. In Cassandra's case, the SEDA architecture allows many concurrent requests per machine (typically on the order of ~128 or ~256 read and write threads per machine), and the distributed nature allows you to scale up with many machines (single clusters on the order of thousands of machines).