Why Is MongoDB Faster Than Postgres?

Problem Why is MongoDB faster than Postgres in some cases?

Be ready to discuss

• The framing to lead with: "faster" is workload-dependent, not universal — the right answer names the access pattern that makes each win, and refusing the premise is part of a good answer.
• Document locality: a whole logical entity (an order with its line items) lives in one BSON document, so a read that Postgres would satisfy with joins across normalized tables becomes a single lookup — fewer random I/Os on denormalized, read-heavy paths.
• Schema flexibility: no ALTER TABLE migration or lock when fields change, which matters for evolving, write-heavy workloads.
• Horizontal scaling: native sharding for write scale-out, versus Postgres needing extensions (Citus) or manual architecture to shard writes across nodes.
• Write path and durability defaults: MongoDB's default write concern and journaling behavior can acknowledge sooner than a synchronous Postgres commit — some measured "speed" is a durability trade, not an engine advantage.
• Where Postgres wins: complex multi-table joins, ACID across many rows, rich SQL analytics with a mature cost-based optimizer, and data integrity enforced by constraints rather than by application code.
• The counterpoint to have ready: the same locality benefit is available in Postgres via JSONB, and MongoDB has had multi-document ACID transactions since 4.0 (at a performance cost) — so the gap is about default fit, not capability.
• Benchmark honesty: comparisons usually differ in schema design, index coverage, and durability settings rather than in raw engine speed.