MongoDB will flush data to the disk every minute, by default. The background flush refers to the time it takes for the data to persist to the disk. It should not be more than 1 second for every 1 minute period.

Modifying the flush interval may help with the background flush time; by writing to the disk more frequently, there will be less data to write. This could, in some cases, make writes faster.

The fact that the background flush time gets affected by the write load means that if our background flush time starts to get too high, we should consider sharding our database to increase the write capacity.