Whenever the case occurs that an index build could occur simultaneously (initial sync, or via oplog using background indexes) applying to the same collection, MMAP builds the indexes in serial, while WiredTiger builds the indexes in parallel (no observed limit).

This has performance implications in both directions.

• MMAP does not make use of available hardware beyond the disk IO for a single index (despite the other indexes potentially requiring the same data).
• WiredTiger can overrun the capacity of a lower powered machine (a hidden secondary for backup or analytic purposes) and end up thrashing to cause significant stalling.

This ticket is to request a review of the WiredTiger behavior to consider if some feedback on the hardware can be taken into consideration when building indexes in parallel.