MongoDB 5.0 made the default write concern majority, which slows uninstrumented writes

Before 5.0 the implicit write concern was w 1, acknowledged by the primary alone. From 5.0 the default is the calculated default write concern, which is majority for most deployments, so a write returns only once it has reached a majority of data-bearing voting members.

Throughput-sensitive ingest paths that never set a write concern therefore slow measurably after the upgrade, and on a degraded replica set without an available majority those writes block until wtimeout rather than returning quickly. Inspect the effective value with the getDefaultRWConcern admin command and change it with setDefaultRWConcern rather than editing every call site. A per-operation writeConcern still overrides the default. Note that majority acknowledgement does not imply reads observe the write; that requires readConcern majority or reading from the primary.