We seem to have a performance problem with $limit in aggregate queries. The query is very very slow (half a minute) - when the number of docs passed into the limit step are less than the limit size.

The query is instantaneous when there are lots of docs returned from stage 1. However, when there are only a few, the aggregate query is very very slow.

Expected behavior: aggregate performance of $limit should be equivalent to a standard query's use of limit.