Improved Performance in Aggregations
The newly implemented slot-based query execution engine improves performance for a wider range of find and aggregation queries. MongoDB can use the slot-based query execution engine for a subset of queries provided certain conditions are met. In most cases, the slot-based execution engine provides improved performance and lower CPU and memory costs compared to the classic query engine.
Compound Wildcard Indexes
You can now create compound wildcard indexes. A compound wildcard index has one wildcard term and one or more additional index terms. Use compound wildcard indexes to support queries on known patterns and to limit the total number of indexes in a collection.
Consider a collection like this: