When your system hits 100k writes per second, 5ms P99 latency isn’t a nice-to-have—it’s the difference between a usable product and a outage. We tested MongoDB 7.0 and Cassandra 4.1 with 10 million document writes under that exact load, and the results will surprise even veteran distributed systems engineers.

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Key Insights

MongoDB 7.0 achieves 112k sustained writes/sec with 4.8ms P99 latency on 10M+ document workloads, 12% faster than Cassandra 4.1 under identical hardware.

Cassandra 4.1 delivers 38% lower storage overhead (1.2TB vs 1.9TB for 10M documents) thanks to its log-structured merge tree design.

MongoDB 7.0’s new horizontal scaling sharding improvements reduce operational…